Physics problem: relative motion?

A boat is traveling upstream at 16 km/h with respect to the water of a river. The water is flowing at 8.0 km/h with respect to the ground. (a) What is the speed of the boat with respect to the ground? (b) A child on the boat walks from front to rear at 4.0 km/h with respect to the boat. What is the child's speed with respect to the ground?