Ace Hardware sells boxes of wrenches ($ 100) and hammers ($300). Howard ordered 40 boxes of wrenches and hammers for $ 8,400. How many boxes of each are in the order.

100w+300h=8400

h+w=40, h=40-w, using this h in the first equation gives you:

100w+300 (40-w) = 8400 perform the indicated multiplication on left side

100w+12000-300w=8400 combine like terms on the left side

-200w+12000=8400 subtract 12000 from both sides

-200w=-3600 divide both sides by - 200

w=18, and since h=40-w

h=40-18=22

So Howard ordered 18 boxes of wrenches and 22 boxes of hammers.