Method 1: conventional

Let the boy have x chocolates initially and let him have y amount of money to buy these chocolates

So with y amount of money he can buy x-8 chocolates and with 1.5y he can buy x+8 chocolates

So  =

Hence x= 40

Method 2: Assumption

If we assume that the boy had 100 rs with him initially

We can now go from the answer option and pick the number of chocolates he had initially. Put in the conditions given and the price of each chocolate has to be the same:

If we assume 40 is the right option

In condition 1 we will have to buy 32 chocolates with 100 rs, where as we will have to buy 48 chocolates with 150 rs. If each of these chocolates cost the same then the assumption is correct.

i.e ,100/32 =150/48 . hence 40 is the right option.