The equation r=10a-20 represents the revenue r (in dollars) you make by spending a dollar on advertising your revenue totaled $110. how much did you spend advertising?

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Your equation is 10a-20=$110 so first you + $20 both sides of the equal sign. So you cancelled out -20 by +20 and you added 20 to $110 so now your equation is 10a=$130 you divide by 10 on both sides and you're left with a=13. Hope this helped tried to make it as clear as possible.