We do like the following:
Set the first number is a and the second is b.
We have two equations: a+2xb=24, 2xa+b=21
And we have 2xa+4xb=48 or 2xa=48-4xb
So: 48-4xb+b=21 or -3xb=21-48=-27 and we got b=-27:(-3)=9 and a= 6
a=6 and b=9
It’s b is the second s is supposed to be 5
10 kilometers is equal to a little over 6 miles. And 6 > 5, therefore 10 kilometers is your answer.