**Hint:** Try to deduce from the question, which is CP and which is SP. Then, if we have SP more than CP, we get gain. Use the fact that gain is equal to selling price minus cost price, i.e. gain = SP - CP. Now, the gain percentage is the amount of gain per 100 units of CP. Now, using the unitary method, find the gain percentage of the student in this whole transaction.

**Complete step by step answer:**

We have the cost at which the apples were purchased by the boy, CP = Rs. 9.60

The amount for which the apples were sold by the student, SP = Rs. 12

Hence the gain earned by the boy SP - CP = Rs 12 - Rs 9.60 = Rs. 2.40

Hence the gain earned by the boy is equal to Rs.2.40

Gain percentage is defined as the amount of gain earned per 100 units of cost price.

We have, the gain on Rs 9.60 of CP = Rs. 2.40

Hence, using unitary method, we have

The gain on Re. 1 of CP $=\dfrac{2.40}{9.60}=\dfrac{1}{4}$

Hence gain per Rs. 100 of CP $=\dfrac{1}{4}\times 100=25\%$

**Hence the gain percentage of the student = 25%**

**Note:** As an alternative method, we can use the direct formula to compute the gain percentage as $gain\%=\dfrac{gain}{C.P}\times 100$ to get the gain% earned by the student in this transaction. So, first we calculate the gain by substituting the values of SP and CP, then we get Rs. 2.40. Now, we will get gain% as

$gain\%=\dfrac{2.40}{9.60}\times 100=25\%$