Jason hired a luxury car service. the service charges a driver rate of $4.92, regardless of number of miles traveled, plus an additional $3.79 per mile. if jason wants to spend no more than $4.20 per mile, what is the minimum number of mile should travel

Let's make two expressions.
4.92+3.79x
This represents the car service's cost, which x being miles.
4.2x 
This is the most Jason wants to spend. Let's make them equal.
4.92+3.79x=4.2x
We need to get x by itself, Let subtract 3.79x from both sides.
4.92=0.41x
Let's divide.
4.92÷0.41=12
So, he should not drive no more than 12 miles.

If the diameter is 8 than the radius is 4 because 2r or 2*radius equals the diameter. 4*2=8. 8/2=4.