for this question you can say:
1/8x = 10
so x is the answer and if u want to find the x you can multiply both sides of the equation by 8 so you will have:
x = 80 and that's the number
if u want to find 1/7 of 21 you should just divide 21 by 7 so you will have 3 and for the last one is same also, so the answer is 5 :)))
i hope this is helpful
have a nice day