A boat can travel 15 km upstream and 18 km downstream in 4 hours.
It can go 18 km upstream and 15 km downstream in 4 hours 15 minutes.
Formula used: Total Time = Distance/(Downstream speed) + Distance/(Upstream speed)
Downstream speed = Speed of the boat in still water + Speed of the stream
Upstream speed = Speed of the boat in still water – Speed of the stream
Calculation: 4 hr 15 min
= hr
Let the speed of the boat be x km/hr and speed of current be y km/hr
According to the question
+=4 Let(x–y)=aand(x+y)=b 15a + 18b = 4 ---- (1)
⇒
+= 4 hr 15 min
⇒
18a+15b= ---(2)
Add equation (1) and equation (2)
33(a+b)= ⇒
a+b= ---(3)
Subtract equation (1) from equation (2)
3(a–b)= ⇒a–b= ---(4)
From equation (1) and equation (2)
a= and
b= ⇒ x - y = 6 ---(5)
⇒ x + y = 12 ---(6)
From equation (1) and equation (2)
x = 9 and y = 3
∴ Speed of boat in still water is = 9 km/hr