Let's just make up some variables.
D = Donatello's age in 2005, L = Leo's
(D + 9) = Donatello's age in 2014.
D = 11L + 1
(D + 9) = 3(L + 9) + 7
We need to get one variable on its own in both equations to use substitution:
D = 11L + 1
D + 9 = 3(L + 9) + 7
D + 9 = 3L + 27 + 7
D = 3L + 27 + 7 - 9
D = 3L + 25
Now we can substitute 3L + 25 for D in the other equation to solve for L.
D = 11L + 1
3L + 25 = 11L + 1
24 = 8L
L = 3
Use L = 3 in an earlier equation to find D.
D = 11L + 1
D = 11*3 + 1
D = 33 + 1
D = 34
Now we know that in 2005, Leo was 3 and Don was 34.
In 2005, Leo was 3 -- true
In 2005, Don was 23 -- fasle
In 2014, Leo was 15 -- fasle (3+9=12)
In 2014, Don was 43 -- true (34+9=43)
Leo is older than Don -- false
Don is older than Leo -- true
