Question 56902
"sally is 3 times older that david"
Let David's present age = X
Then Sally's age = 3X

" in five years" 
David's age = X+5
Sally's age = 3X+5

"sally will be twice as old as david"
3X+5 = 2(X+5)
Solving this
3X+5 = 2X+10
X = 5 
Which means David's present age = 5
Sally's present age = 15

"how old will sally be in 5 years"
15+5 = 20 years