Question

If Kurt were 10 years old, he would be twise as old as his sister, and their combine ages would be 36. How old are Kurt and his sister?
Reiny
Did you mean "If Kurt was 10 years older" ?
The way you have it, Kurt is 10, his sister would have to be 5, but then their combined ages can't be 36

If as I think it should be,
let Kurt's present age be x
let his sister's present age by y

in 10 years ...
Kurt = x+10
sister = y +10
we are told that Kurt is twice her age at that time ...
x+10 = 2(y+10)
x = 2y + 10

also x+10 + y+10 = 36
x + y = 16
sub in x = 2y+10
2y+10 + y = 16
3y = 6
y = 2 , then x = 14

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