A boat travels 30 km upstream in the same amount of time it takes to travel 60 km downstream in the same river. If the speed of the river is 4 km/h, what is the speed of the boat in still water​?

From the given situation in the question above,

let us assume that 

speed of the boat in still water = x km/hr

Given speed of the river, v = 4km/hr

so when boat is going upstream ,speed of boat will be, 

v₁ = x-4 km/hr

and when it is going downstream, the speed will be, 

v₂ = x+4 km/hr

Distance travelled during upstream, d₁ = 30km

Distance travelled during downstream, d₂ = 60km

Let us assume that, 

t₁ = times taken to travel upstream

t₂ = time taken to travel downstream

using the formula , time = distance/speed

t₁ =  d₁/v₁ = 30/x-4

t₂= d₂/v₂ = 60/x+4

But time is equal as given in question, t₁=t₂

1(x+4) = 2(x-4)

x+4 = 2x - 8 

x = 12

Speed of boat in still water is 12 km/hr