life expectancy is defined by the term 2SB/G

[griffin.811]

5th ed. Algebra strategy guide, ch.6 Formulas problem set, Q.4, p.85:

Life expectancy is defined by the formula 2SB/G where S= shoe size, B=average monthly electric bill in dollars, and G = GMAT Score. If Melvin's GMAT score is twice his monthly bill, and his life expectancy is 50, what is his shoe size?

Answer: S=50

my Q: why doesn't S=25? or better yet, it seems that in finding the solution s=50, the book doesnt distribute the 2 across both the S and the B, why isn't that the case?

When I solve I get: 2SB/2B=50 --> (2s)(2b)/2b=50 -->2s=50 --> S=25.

where am I going wrong here?

Thanks

[RonPurewal]

2SB is not 2S times 2B. Because ... well, because it isn't.
(:
(The left-hand expression is the product of 2, S, and B. The right-hand expression is the product of 2, S, another 2, and B.)

Try throwing in a few pairs of actual numbers for S and B, and calculating the values of both products.
It will be abundantly clear that they are not the same.