Question

Please help. Kristen can run 15km h^-1 and walk at 5km h^h-1. She completed a 42 km marathon in 4 hours. What distance did Kristen run during the marathon?

Answer 1

15x + 5y = 42
x + y = 4 (times this by five so you can get rid of one value)

15x + 5y = 42
-
5x + 5y = 20

= 10x = 22
x = 2.2 (sub back into original)
y = 1.8

Answer 2

Of course, this all assumes she didn't spend any of that 4 hours at any other speed, or stopped, which isn't very realistic :-) I agree with jordirh's result, however.