If it was 60 miles round trip, each way was therefore 30 miles.
Let x represent the hours taken on the dry road. Slippery road was then 2x. 2x + x = 3x. 35 / 3 = 11.66… = x.
So, on the dry road, the car took 11.66… hours.
For the slippery one, 2 * 11.66… or 23.33… hours.
Convert to mph, shall we?
Dry road:
30 miles / 11.66… hours = 2.57… mph.
Slippery road:
30 miles / 23.33… hours = 1.28… mph.
If that’s what you’re looking for — you never quite asked.