5th Edition Word Problems Chapter 1, Problem Set: Question 1

[joe_mals]

Hi Everyone,

Here is Question: John is 20 years older than Brian. Twelve years ago, John was twice as old as Brian. How old is brian.

How come if I try to solve the problem by combination instead of substitution, I come up with the wrong answer? Am I not allowed to use combination in this problem for some reason?

J = 20 + B
J - 12 = 2(B - 12)


Substitution (as the book does it):

(b+20) - 12 = 2B - 24
b + 8 = 2b - 24
b= 32


Combination: (Where am I wrong here?)

J = B + 20 -----> 2J = 2B + 40
J= 2B -12


When you subtract the two equations you get

J= 28

When you plug back into J = 20 + B, B=8 which is wrong


Thanks

[RonPurewal]

40 minus -12 is not 28. Watch your signs.