M03 Q7

Your doubt: Why we have multiplied 2 with {customers who by exactly 3 products} and subtracted by {customer who buy exactly 2 products} ?
-- This is because Once you add up "{customers who buy milk}+{customers who buy chicken}+{customers who buy apples}" you are already double counting {customer who buy exactly 2 products} and triple counting {customers who by exactly 3 products}. Hence you subtract the additional ones.

Let’s analyze above based on what is provided in the problem. Assume total food mart customers as 100.
{customers who buy milk}+{customers who buy chicken}+{customers who buy apples} = 60 + 50 + 35 = 145

In our diagram, it can be represented as
(a + d + g + e) + (b + d + g + f) + (c + e + g + f) = 145
(a + b + c + d + e + f + g) + (d + e + f) + 2g = 145
From the diagram, a + b + c + d + e + f + g = 100
100 + (d + e + f) + 2g = 145
100 = 145 - (d + e + f) - 2g
i.e. 100 = (milk customers + chicken customers + apple customers) - (customers who buy exactly 2 products) - 2 * (customers who buy exactly 3 products)
-- This answers your doubt.

Now lets solve the problem further
100 = 145 - (d + e + f) - 2g
(d + e + f) + 2g = 45
We know g=10,
Hence, customers who purchase exactly 2 products = (d + e + f) = 25

Thus choice(D) is the answer