C) explain why David's average speed is less than his top speed.​

Explanation:

His top speed is a blip that occurs for less than the full time he was out. Below is a graph showing what he did from 1 o'clock to 9. He was walking from 1 to the point where starts walking faster and then back down again to 2. The line at 2 is longer than the triangular shape that shows where he speeded up. Then he went back down to 2 and continued at that speed until 9.

Obviously he spent more time at 2 than he did at 4 and therefore his average speed will less than the 4 speed.

Remember that average speed = total distance / total time.

answer: 2×10^-2kg

explanation: density= mass per volume

vol=mass/density

converting to s.i units we have;

3÷1000kg = 0.003

1.5÷100m = 0.15

0.003/0.15 = 0.02kg