# GMAT math thinking skills – 9

GMAT tests your logical skills as well as your knowledge of math concepts.  To score high, you need to remember various formulas, theorems. Also you need to master critical problem-solving skills.

Today I am going to  take you through one problem -solving skill –

## Take this problem .

I am going to show you, two ways to solve this sum

Method 1 :  The traditional approach

You need to choose the numbers which negate each other

For example Row 6 column 6 = 36.

Row 5 column 6 = -30 and  Row 1 column 6 is -6

So 36+ (-30-6) =0.

You need to choose the right numbers

I have highlighted the different combinations with letters

I have highlighted the different combinations with letters

Numbers left =  12 15  -6 -10 -2

Sum = 9

This method takes a long time

Method 2: Pattern recognition

This approach is a lot faster.

If you look at the numbers column-wise can you see a pattern(ignore signs)

column 1: 1 2,3,4,5,6

column 2: 2×1,2×2,2×3,2×4,2×5,2×6

.

.

column 6: 6×1,6×2,6×3………..6×6

When the signs are inserted, then a pattern emerges  i.e. = 1-2+3-4+5-6

lets calculate the sum of the repeating pattern 1-2+3-4+5-6 = -3

Hence column 1 adds upto -3

Next column will be -2+4-6+8-10+12 = -2 x(1-2+3-4+5-6)= -2x-3 = 6

Next column will be 3 x(1-2+3-4+5-6)= 3 x-3 = -9

Next column will be -4 x(1-2+3-4+5-6)= -4 x-3 = 12

Next column will be 5 x(1-2+3-4+5-6)= 5 x-3 = -15

Next column will be -6 x(1-2+3-4+5-6)= -6 x-3 = +18

Answer = -3+6-9 +12-15+18 = 9

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