Bhanu travels from City A to City B. If Bhanu drives his car 3/7 at of his normal speed, then he reaches City B 28 minutes late. Find the time (in minutes) that Bhanu would have taken to travel from City A to City B if he drove at his normal speed.

Correct option is C

Given:

Bhanu drives at $$\frac{3}{7}$$​ of his normal speed.

Bhanu is 28 minutes late when driving at this reduced speed.

We need to find the time it would take Bhanu to travel from City A to City B if he drove at his normal speed.

Concept Used:

The time taken to travel is inversely proportional to the speed, meaning that if speed decreases, time increases.

Solution: 

Let T be the time Bhanu would take at his normal speed and S be Bhanu’s normal speed.

When Bhanu drives at  $$\frac{3}{7}$$​ of his normal speed, the time taken will increase by a factor of  $$\frac{7}{3}$$​​

The time taken at reduced speed is: $$\frac{7}{3} \times T$$​​

The difference in time is 28 minutes:

$$\frac{7}{3} \times T - T = 28$$​​

​$$\left( \frac{7}{3} - 1 \right) \times T = 28 \\ \ \\\frac{4}{3} \times T = 28 \\ \ \\T = 28 \times \frac{3}{4} = 21$$​​

Thus, the time Bhanu would have taken to travel from City A to City B at his normal speed is 21 minutes 

Alternate Solution: ​

For constant distance; 

Speed $$\propto \frac1{Time}$$ 

Speed ratio = 7    :     3

Time ratio = 3     :     7

7 - 3 = 28 min.

4 unit = 28 min.

1 unit = 7 min. 

time taken by normal speed = 3 × 7 = 21 min.