Question

The speed of a boat in still water is 12 km/h. If the boat covers a distance of 38 km upstream in 4 hours then the speed of the stream (in km/h) is:

Answer 1

Mathematics beauty is to known unknown, let's assume speed of stream be x km/h.

• Upstream: (12 - x) km/h

We know that t = d/s

→ 38/(12 - x) = 4

→ 38 = 4(12 - x)

→ 38 = 48 - 4x

→ 4x = 48 - 38

→ 4x = 10

→ x = 2.5 km/h

Hence speed of stream is 2.5 km/h

Answer 2

$\bold{\underline{\underline{Answer:}}}$

Speed of stream = 2.5 km/hr

$\bold{\underline{\underline{Stpe\:by\:step\:explanation:}}}$

Given :

• The speed of a boat in still water is 12 km/h
• The boat covers a distance of 38 km upstream
• Time = 4 hours

Solution :

Let the speed of the stream be x km/hr.

Speed of boat upstream = (12 - x) km/ hr.

We have speed (with an unknown quantity x), time and distance.

Therefore using the formula :-

$\bold{\large{\boxed{\rm{\red{Speed\:=\:{\dfrac{Distance}{Time}}}}}}}$

Block in the values,

$\rightarrow$$\bold{(12-x)}$ = $\bold{\dfrac{38}{4}}$

Cross multiplying,

$\rightarrow$$\bold{4(12-x)=38}$

$\rightarrow$$\bold{48-4x=38}$

$\rightarrow$$\bold{-4x=38-48}$

$\rightarrow$$\bold{-4x=-10}$

$\rightarrow$$\bold{x={\dfrac{-10}{-4}}}$

$\rightarrow$$\bold{x={\dfrac{10}{4}}}$

$\rightarrow$$\bold{x=2.5}$

•°• Speed of stream, x = 2.5 km/hr