Easy word problems. I can't seem to remember how to solve?

Please post the picture right-side up!

It takes Rosa r hours to do the job alone.
She does 1/r of the job per hour.

It takes Dianne r-8 hours to do the job alone.
She does 1/(r-8) of the job per hour.

Working together, they do 1/r + 1/(r-8) of the job per hour.
Since it take them 3 hours to finish, they do ⅓ of the job per hour.
1/r + 1/(r-8) = ⅓
r² - 14r + 24 = 0
r = 2,12
2 is an extraneous solution, so r = 12

It takes Rosa 12 hours to do the job alone.
:::::
Ariel drives a miles per hour.
Sharon drives a-15 miles per hour.

100 miles × (1 hour)/(a miles) = 100/a hours
It takes Ariel 100/a hours to drive 100 miles.

80 miles × (1hour)/(a-15 miles) = 80/(a-15) hours
It takes Sharon 80/(a-15) hours to drive 80 miles.

100/a = 80/(a-15)
100(a-15) = 80a
20a = 1500
a = 75
Ariel drives 75 miles per hour.