The speed of the boat along the river is 17 1/2 km per hour, and against the current 12 1/2 km per hour.

The speed of the boat along the river is 17 1/2 km per hour, and against the current 12 1/2 km per hour. What is the speed of the current?

Take for:
x – boat speed
y – flow rate
Given:
x + y = 17 1/2 km / h
x – y = 12 1/2 km / h
Find y – the speed of the river
Decision:
Let’s make a system:
x + y = 17 1/2 km / h
x – y = 12 1/2 km / h
Means
x + y + (x – y) = 17 1/2 km / h + 12 1/2 km / h = 30 km / h
2x = 30 km / h
x = 15 km / h
from any of the equations we find that
y = 17 1/2 km / h – 15 km / h = 2 1/2 km / h
Answer: current speed 2 1/2 km / h