A swimmer heads directly across a river, swimming at 1.5 m/s relative to the water. She arrives at a point 30 m downstream from the point directly across the river, which is 87 m wide.

Answer 1

It takes her 87/1.5 seconds to cross
the current flows 30 meters in 87/1.5 seconds
so the current speed is 30 *1.5 /87