Question

Ritu can row downstream 20km in 2 hours and upstream 4km in 2 hours. Find her speed of rowing in still water and speed of current.

Answer 1

$\huge\star{\green{\underline{\mathfrak{Answer :-}}}}$

Given :

• Row downstream 20 km in 2 hrs.
• Row upstream 4 km in 2 hrs.

To find :

• Speed of rowing in still water
• Speed of current

Solution :

Let the speed of boat in still water be X km/hr and speed of still water be y km/hr.

We know that,

Speed downstream = x + y

Speed unstream = x - y

By using the given we can write;

For upstream,

$\mapsto\tt{x + y = \dfrac{20}{2}}$

$\mapsto\tt{\pink{x + y = 10}}$

For downstream,

$\mapsto\tt{x - y = \dfrac{4}{2}}$

$\mapsto\tt{\pink{x - y = 2}}$

With elimination method,

$\tt{x \cancel{+ y} = 10}$

$\tt{x \cancel{- y} = 2}$

$\rule{50}1$

$\tt{\purple{2x = 12}}$

$\mapsto\tt{\green{x = 6}}$

So, the speed of boat in still water is 6 km/hr.

Now,

$\mapsto\tt{x - y = 2}$

$\mapsto\tt{6 - y = 2}$

$\mapsto\tt{- y = 2 - 6}$

$\mapsto\tt{\green{y = 4}}$

So, the speed of current is 4 km/hr.

$\rule{200}4$

Answer 2

Given: Ritu can row -

• downstream 20 km in 2 hours.
• upstream 4 km in 2 hours.

To find: Her speed of rowing in still water and the speed of the current.

Answer:

Let the speed of the boat in still water be x km/h, and that of the current be y km/h.

Therefore, speed downstream would be x + y km/h and speed upstream would be x - y km/h.

According to the question,

$\sf x\ +\ y\ =\ \dfrac{20}{2}\ and\ x\ -\ y\ =\ \dfrac{4}{2}.$

On further simplification,

x + y = 10 and x - y = 2.

On solving these equations, we get x = 6 and y = 4.

Therefore, the speed of the boat in still water is 6 km/h and the speed of the current is 4 km/h.