Kristen can run at 15 km/h and walk at 5 km/h. she complete for a 42 km marathon in 4 hours. what distance did kristen run during the marathon?

Let x be the time she ran and y be the time she walked 

then distance ran =  25x and distance walked = 5y 

so 15x + 5y = 42

and x + y = 4

* second equn. by -5:-

-5x - 5y = -20

add this to the first equn:0

10x = 22
x = 2.2

distance she ran = 2.2 * 15  =  33 km