Calculate Downstream and Upstream speeds

Downstream speed is the sum of boat speed and current speed. Upstream speed is the difference between boat speed and current speed.

$V_{down} = 24 + 21 = 45 \text{ km/h}, \quad V_{up} = 24 - 21 = 3 \text{ km/h}$

Calculate time for each direction

Using the formula $T = D/V$, calculate time for downstream ($T_d$) and upstream ($T_u$) journeys separately.

$T_d = \frac{270}{45} = 6 \text{ hours}, \quad T_u = \frac{270}{3} = 90 \text{ hours}$

Calculate total time

Sum the time taken for both directions to find the total time taken by the boat.

$T_{total} = 6 + 90 = 96 \text{ hours}$