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