I don't think that such kind of problems appear in GMAT.

We can use this formula to solve this one: \(\frac{m(m+1)*n(n+1)}{4}\)==> \(\frac{4*5*5*6}{4}\)=150

Alternative solution:

In a rectangle m × n there are (m+1) vertical grid lines

and (n+1) horizontal grid lines (5 and in the example here).

To define any rectangle within the grid,

we must choose 2 of each and there are

( (m+1) choose 2 ) × ( (n+1) choose 2 ) ways to do that.

For 5 × 6 that gives us

2C5 × 2C6 = 10 * 21 = 150

What would the experts say about this question, isn't it too specific/hard for GMAT ?

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