Question

In one hour , a boat goes 11 km/hr along the stream and 5 km/hr against the stream . The speed of the boat in still water ( in km/hr) is :

• A. 3 km/hr
• B. 5 km/hr
• C. 8 km/hr
• D. 9 km/hr

Answer 1

Answer:

C. 8 km/hr

• Speed Downstream ($D$): 11 km/hr (along the stream)

• Speed Upstream ($U$): 5 km/hr (against the stream)

When a boat moves in water, its actual speed is affected by the current:

• Downstream Speed: Speed of boat ($x$) + Speed of stream ($y$)

• Upstream Speed: Speed of boat ($x$) - Speed of stream ($y$)

The formula for the speed of the boat in still water ($x$) is the average of the downstream and upstream speeds:

$x = \frac{D + U}{2}$

Substituting the values:

$x = \frac{11 + 5}{2}$

$x = \frac{16}{2}$

$x = 8 \text{ km/hr}$