B C H * G E I --------------- B A D B A A I J ...
* G E I
---------------
B A D B
A A I J
A F F F
----------------
A H J F D B
Let's start this way:
B C H
* G E I
---------------
B A D B
A A I J
A F F F
----------------
A H J F D B
J will be 0.
B C H
* G E I
---------------
B A D B
A A I 0
A F F F
----------------
A H 0 F D B
Now focus here:
B C H
* G E I
---------------
B A D B
A A I 0
A F F F
----------------
A H 0 F D B
E * H gives a result whose unit place digit is 0. Which means either one of them is 5 and other is an even digit or vice versa.
So we have two possible cases:
H = 5 and E is even
OR
E = 5 and H is even
Let's proceed with the first case:
Case I: H = 5 and E is even
First of all check whether H = 5 is possible or not
We will try to use the following symmetry in some way. Let's see how can we use this:
B C H
* G E I
---------------
B A D B
A A I 0
A F F F
----------------
A H 0 F D B
Look carefully:
B C H
* G E I
---------------
B A D B
A A I 0
A F F F
----------------
A H 0 F D B
A + F + (Some Carry from last step) = H
A + F + (Some Carry from last step) = 5(as we are proceeding for H = 5) -----------(1)
What do you think will be the carry from last step ? Let's see this:
B C H
* G E I
---------------
B A D B
A A I 0
A F F F
----------------
A H 0 F D B
B + A + F + (Some carry from last step) = X0 (X is carry, just ignore it) -------------(2)
And what you think will be carry from the last step ?
Let's see that too:
B C H
* G E I
---------------
B A D B
A A I 0
A F F F
----------------
A H 0 F D B
A + I + F + (Some carry from last step)= XF (X is carry, just ignore it)
The last step D + 0 = D will not produce any carry. So In this case carry will be 0.
which implies:
A + I + F = XF => A + I will be 10
So, A + I + F will produce carry 1
Putting this carry in the equation (2), we get:
B + A + F + 1 = X0 ---------------(3)
Also from equation (1) we had: A + F + (Some Carry from last step) = 5, which means that A + F cannot be more than 5.
So, B + A + F + 1 cannot be other than 10, which means this step produces a carry 1.
Putting this carry in eq (1) we get:
A + F = 4
Substituting this value of (A + F) in eq (3):
B + 4 + 1 = X0 => B will be 5
WAIT !! What we got ?? We assumed H = 5 and following that we got B = 5. Not acceptable. Reject this solution and pick the other option.
Case II: E = 5 and H is even
B C H
* G 5 I
---------------
B A D B
A A I 0
A F F F
----------------
A H 0 F D B
Possible values of H = {2, 4, 6, 8}
Let's start one by one:
When H = 2:
B C 2
* G 5 I
---------------
B A D B
A A I 0
A F F F
----------------
A 2 0 F D B
First check whether H = 2 is possible or not.
B C 2
* G 5 I
---------------
B A D B
A A I 0
A F F F
----------------
A 2 0 F D B
A + F + (some carry from last step) = 2
Clearly the last step is producing some carry as the result is having unit place digit 0.
A + F + (some non zero carry from last step) = 2
Sum of three positive integers can't be 2(no one can be 0 because 0 is already there)
So H = 2 is not possible.
When H = 4:
B C 4
* G 5 I
---------------
B A D B
A A I 0
A F F F
----------------
A 4 0 F D B
Follow the same step we followed in case I when we rejected H = 2. You can easily find B = 6.
6 C 4
* G 5 I
---------------
6 A D 6
A A I 0
A F F F
----------------
A 4 0 F D 6
Notice:
6 C 4
* G 5 I
---------------
6 A D 6
A A I 0
A F F F
----------------
A 4 0 F D 6
I can be either 4 or 9. But 4 is already there so I can be 9 only
6 C 4
* G 5 9
---------------
6 A D 6
A A 9 0
A F F F
----------------
A 4 0 F D 6
Look carefully:
6 C 4
* G 5 9
---------------
6 A D 6
A A 9 0
A F F F
----------------
A 4 0 F D 6
5 * 4 = 20, so carry is 2. Now in the next multiplication 5 * C is giving unit place digit 9.
Is, 5 * C + 2 = X9 possible ? (X is just a carry, ignore it)
In this case also we are getting wrong result. So we will move to the next case.
When H = 6:
B C 6
* G 5 I
---------------
B A D
A A I 0
A F F F
----------------
A 6 0 F D B
Follow the same step we followed in case I when we rejected H = 2. You can easily find B = 4.
4 C 6
* G 5 I
---------------
4 A D 4
A A I 0
A F F F
----------------
A 6 0 F D 4
Focus here:
4 C 6
* G 5 I
---------------
4 A D 4
A A I 0
A F F F
----------------
A 6 0 F D 4
I can be none other than 9
4 C 6
* G 5 9
---------------
4 A D 4
A A 9 0
A F F F
----------------
A 6 0 F D 4
Again because of the same reason as in previous case,
4 C 6
* G 5 9
---------------
4 A D 4
A A 9 0
A F F F
----------------
A 6 0 F D 4
5 * C + 3 = X9 is not possible, so we can reject this case also.
Now we are left with just one case H = 8
Let's proceed with that too.
When H = 8:
B C 8
* G 5 I
---------------
B A D
A A I 0
A F F F
----------------
A 8 0 F D B
Follow the same step we followed in case I when we rejected H = 2. You can easily find B = 2.
2 C 8
* G 5 I
---------------
2 A D 2
A A I 0
A F F F
----------------
A 8 0 F D 2
Look at the highlighted characters:
2 C 8
* G 5 I
---------------
2 A D 2
A A I 0
A F F F
----------------
A 8 0 F D 2
Clearly I can be either 4 or 9
When I = 4:
2 C 8
* G 5 4
---------------
2 A D 2
A A 4 0
A F F F
----------------
A 8 0 F D 2
Again check if this value of 4 is acceptable or not
2 C 8
* G 5 4
---------------
2 A D 2
A A 4 0
A F F F
----------------
A 8 0 F D 2
So you think 4 * 2 + (some carry from last step) can produce a result like 2A. Its never possible. So we reject this case also.
Now the last case when I is 9.
When I = 9:
2 C 8
* G 5 9
---------------
2 A D 2
A A 9 0
A F F F
----------------
A 8 0 F D 2
See the highlighted characters:
See the highlighted characters:
2 C 8
* G 5 9
---------------
2 A D 2
A A 9 0
A F F F
----------------
A 8 0 F D 2
A + 9 + F = F (No carry from the last step)
=> A + 9 = 10
=> A = 1
2 C 8
* G 5 9
---------------
2 1 D 2
1 1 9 0
1 F F F
----------------
1 8 0 F D 2
Now you have everything in your hand. You can easily reach to the solution:
2 3 8
* 7 5 9
---------------
2 1 4 2
1 1 9 0
1 6 6 6
----------------
1 8 0 6 4 2
You must be thinking its too long. I have explained all the steps thats why it went so long. When you will be practicing a lot, you will be in a position to solve these problems in not more than 15 minutes. Solving 3 question in eLitmus in 15 minutes is worth. Don't leave this problem. It just needs some practice and nothing else.