Problem Solving

An alchemist discovered that the formula to turn ordinary metal into gold is G = M + 15, where G is the number of gold bars and M is the number of metal bars. If a metal bar weighs twice as much as a gold bar, how many metal bars will yield an equal weight of gold?
10
15
22.5
30
67.5

You can use reverse Plugging in to bypass the confusing algebra: The 5 answers are supposed to be the number of metal bars. Let's assume that a metal bar weighs 2 kilos, twice as much as a gold bar which weighs 1 kilo. Go down the answer choices, plug in the answer as the number of metal bars, add 15 to find the resulting number of gold bars, and figure out whether the weight of both commodities is the same for this answer choice.

10 metal bars are 10+15=25 gold bars = 25 kilos. But 10 metal bars weight 10*2=20 kilos. Not the same weight. Eliminate.

15 metal bars are 15+15=30 gold bars = 30 kilos. 15 metal bars weight 10*2=30 kilos. Same weight of 30 kilos - we're done. Choose B.