Step-by-step explanation:
1 t=16/21
2.m=2
3.n=13/7
4.a=2
5.x=6/17
6.x=15
7.s=21/4
8. t=7/3
9. s=1
10. s=6/61
11. x=1/3
12. r=27/16
13. c=−1
14.m=9/5n
15. j=−117/58
Well, their speeds are (

is Jack's speed, and

is Richard's.

They, together, can paint 12 houses in 35 days. To get a single house, we only have to calculate

which is very close to 3 (a bit below)
139.50/55.8=t
t=2.5 yrs simple interest