In a school experiment, students timed each other as

 

­imRaj
In this question, what assumption says is that 
D2 - D1 / V2 - V1 is constant (3m ~ 15m) For example, for Hopping (10 - 5)/(3.1 - 2.0) = 5/0.9 is constant 

The average speed for hopping 3 meters would be greater than that for speed walking 3 meters.
=> for Speed walking average speed 3m, we can calculate this way 
(2.4 - V) / (5 - 3) = (2.6 - 2.4)/(10-5) result is 2.32. But, average speed for hopping 3 meters will be less than 2m/s 
So, No

The average speed for speed walking 15 meters would be greater than that for hopping 15 meters.
=> average speed for speed walking 15 meters is 2.8 and verage speed for hopping 15 meters is 4.2  
So, No

The average speed for walking backward 15 meters would be greater than that for walking forward 3 meters.
=> average speed for walking backward 15 meters is 2.0 and average speed for walking forward 3 meters is 1.52 
(5-3)/(1.6 - x) = (10-5)/(1.8-1.6)
So, Yes