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
