The time spent to study history and chemistry is 4 hours 30 minutes, Then the student studies physics for:

$$\eqalign{ & \text{Time spend in H + C} \cr & = \left(15 + 15 \right)\% \cr & = 30\% \cr & \text{Time spend in P = 20%} \cr & \because 30\% = 4\frac{1}{2} \text{H} = \frac{9}{2}\text{H} \cr & 1\% = \frac{9}{2} \times 30\text{H} \cr & \text{So, } 20\% = \frac{9}{2} \times \frac{20}{30}\text{H} = 3\text{ Hours} \cr & \text{So } 20\% = \frac{3}{20} \times 20 = 3\text{ Hours} \cr} $$