Mario can run 6 miles in 50 minutes. at this rate how many miles can he run in 30 minutes?

Comment
You are not always obligated to change minutes to hours, especially in a question like this one where times are both given in minutes, and there are only 2 times given.

Givens
d1 = 6 miles
t1 = 50  minutes
r = ???

Equation and solution.
r = d1/t1
r = 6 / 50
r = 0.12 miles / minute. The rate is the same for the next part.

Part 2
Givens
t2 = 30 minutes
r = 0.12 miles/minute
d2 = ???

d = r * t2
d = 0.12 miles/minute * 30 minutes
d = 3.6 miles Note the minutes cancel.

Comment.
There is a quicker way of doing this. The two rates are the same, so you could create a proportion.

d1/t1 = d2/t2
d1 = 6 miles
t1 = 50 minutes
d2 = ???
t2 = 30 minutes

6/50 = d2/30 Cross miltiply
180 = 50 * d2 Divide by 50
180/50 = d2
3.6 = d2

Answer 3.6 miles