by mschwrtz Fri Jun 04, 2010 3:04 am
Good question. The reason you can't see how to infer J = B + 13 from J + 8 = 2B + 16 is that it's impossible to do so. But the text doesn't draw that inference either.
Look back to p. 80-81. Remember that this problem included two equations, (J + 8) = 2(B + 8) AND J = B + 13.
On page 82, after the text rewrites the first of those equations as J + 8 = 2B + 16, it reintroduces the second of those equations. This allows you to use one equation to solve for B in terms of J, and then to plug that expression in terms of J into the other equation.
The line that reads "J = B + 13 --> J - 13 = B" is not a reworking of the line immediately above it. Rather, it is a reintroduction of the equation from the previous page.