Eishan has the right thought process, but made a mistake there.

Number of Prime numbers and Zero: 0, 1, 2, 3, 5, 7 = 6 numbers.

So you have 6 * 5 * 4 options (within prime + zero)=120. 

And 10 * 9 * 8 = 720 options in total.

So 120 / 720 = 1/6.

For question two, Eishan is correct. You have to add 65 mph to 55 mph = 120 mph. With an hour having 60 minutes, that's 2 miles per minute.

To be 20 miles away again, they need to travel 40 miles (20 miles to meet, and another 20 miles). If they travel 40 miles at 2 miles per minute, that's 20 minutes.