math
posted by jack on .
DQ sells 6 times as many orders of french fries as sonic everyday. If both sold 60 more orders DQ would only sell 3 times as many fries. How many do each sell before and after the incease?

S = number FF Sonic sells.
D = number FF DQ sells.
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D = 6S every day.
If each sells 60 more, then
D + 60 = 3(S + 60) or
D + 60 = 3S + 120 or
D = 3S + 120
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The two equators are as follows:
D = 6S
D = 3S + 120
Solve these two simultaneous equation for D and S (before the addition of 60), then add 60 to each to determine the number sold after adding 60 to each. Post your work if you get stuck. 
Where did the 120 come from?