You can find the solution if you identify
- What you are trying to find
- What you know
- How the different quantities are related to each other.
Let's do that with your question:
- You want to find the distance, so let's call it D.
- You know that if Cary goes 60 mph, he gets there 1 hour early AND if he goes 30 mph, he gets there 1 hour late. So if he goes 30 mph, he gets there 2 hours later than if he goes 60 mph.
- Distance = speed x time. Let's write this as D = ST where T will be the time it takes him to get there if he goes 60 mph.
Now you fill in the relationship(s) in part 3 with the values in part 2:
D = 60T
D = 30(T+2)
Because he goes the same distance D, but the time is 2 hours more than the time for the faster speed.
So now you have two equations with two unknown values - only one of which you really care about. To solve this, you need to eliminate the other variable (T). There are several ways of doing this, but I often find the easiest way is to solve both equations for the variable you don't care about first:
D = 60T therefore T = D/60,
D = 30(T+2) therefore D = 30T + 60
T = D/30 - 2
Now remember that when two things equal a third thing, then the first two things have to equal each other, so
T = D/60 and T = D/30 - 2
D/60 = D/30 - 2
and the rest I'll leave to you.
Hope this explanation helps,
Stephen La Rocque.