Kate rode her bicycle for 30 minutes at a speed of 16 mph, then walked for 90 minutes at a speed of 4 mph. What was her overall average speed in miles per hour?

Average speed ==Total distance / total time.

16 x 30/60 ==15 miles travelled on bicycle

4 x 90/60 == 6 miles covered by walking

15 + 6 ==21 miles - total distance travelled.

30/60 + 90/60 ==2 hours - total time

21 / 2 ==10.50 mph - here average speed for the entire trip.