A bird can fly 35 mph in still air 30 mph with the wind and

A bird can fly 35 mph in still air, 30 mph with the wind and 5 mph against the wind. What speed is the wind?

Solution

bird . . . .35 = r
bird . . . 65 = r + c
bird . . . 5 = r -c
substitute r with 35.
65 = 35 + c
c = 30.
check
5 = 35 - 30 [check]
the speed of the wind is 30 mph