With questions like these you must use the information you get to form equations which model your problem.
I'll use these variable in my equation:
- B = big number
- s = small number
B - s = 70 <- big number is first since the difference is positive
B = 8s. <- One number is 8 times another number, well it wouldn't make sense if the smallest number is 8 times the biggest number. So we know it must be B that is 8 times "s".
I can substitute what I have in the second equation for B in the first.
8s - s = 70.
7s = 70
7s/7 = 70/7
s = 10.
So the small number is 10.
B = 10*8
B = 80.
And the big number is 80.