75%
Hope this helps :D
I can also explain if you need help
Hello,
let's assume j the john's age, m the mary's age.
<span>"John is twice as old as mary"==>j=2*m
</span>
<span>"the sum of their ages is 21" ==>j+m=21
==>2*m+m=21
==>3m=21
==>m=7
and j=21-7=14
</span>
Hope this helps helps with the answer to your question ;)