Question

A man can row upstream 36 km in 6 hours. If the speed
of man in still water is 8 km/h, find how much he can
go downstream in 10 hours.
1) 100 km 2) 110 km 3) 90 km
4) 115 km​

Answer 1

$\large\underline{\bf{Solution-}}$

• Let the speed of stream be 'x' km per hour.

It is given that,

• Speed of man in still water = 8 km/ hour.

So,

• Speed of upstream = 8 - x km per hour.

and

• Speed of downstream = 8 + x km per hour.

According to statement,

• Distance covered in upstream = 36 km

• Time = 6 hours

• Speed of upstream = 8 - x km per hour.

We know,

• Distance = Speed × Time

$\rm :\longmapsto\:36 = 6 \times (8 - x)$

$\rm :\longmapsto\:36 = 48 - 6x$

$\rm :\longmapsto\:36 - 48 = - 6x$

$\rm :\longmapsto\: - \: 12 = - \: 6x$

$\bf\implies \:x \: = \: 2$

Thus,

• Speed of stream = 2 km per hour

Now,

• Speed of man in still water = 8 km per hour

• Speed of stream = 2 km per hour

• Speed of downstream = 8 + 2 = 10 km per hour

• Time = 10 hours

Let

• Distance travelled in downstream be 'x' km per hour.

We know,

Distance = Speed × Time

$\rm :\longmapsto\:x = 10 \times 10$

$\bf\implies \:x = 100 \: km$

Thus,

• Distance covered in downstream in 10 hours = 100 km

$\: \: \: \: \: \: \: \: \: \: \: \: \: \pink{ \boxed{ \bf \: Option \: (1) \: is \: correct}}$