1/8 of a number is 10. what is the number?

also..

what is 1/7 of 21 takeaway 1/7 of 35?

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 

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