Billy and
Sally
The
difficulty is to go from English to algebra. Here
is what I make of that long sentence. Sally is S years old; Billy is B
years
old. Billy was once S years old. How long ago was that? It was B-S
years ago.
How old was Sally then? She was S-(B-S) or 2S-B years old then. Billy
is now
twice that old: B = 2(2S-B) = 4S-2B or 3B = 4S. And we also know that
S+B=28.
We have two equations and two unknowns, we can probably solve for S and
B.
Sally is 12 and Billy is 16.
B=Billy's
age S=Sally's age
B+S=28
B=2*(S-x)
(B-x)=S
B=S+x=2S-2x
S=3x
B=2*(3x-x)=4x
4x+3x=28
x=4
B=16=Billy's age
S=12=Sally's age