That's definitely an example of exponential decay, since the base (1/2) (also called the "common ratio") is greater than 0 but less than 1.
Answer:
If w represented the number of weeks he does his chores, the equations would be:
Miles: 120+10w
Simon:75+15w
15 and one half is 31/2. 3 and seven eighths is 31/8. 31/2 also equals 124/8. Then the answer is 124/8 - 31/8 is 91/8. Hope that helped.
Congruent
SSS (because of Pythagoras)