Question 502502
Hi, i am having trouble figuring out the formula used for this type of problem:
 
 A movie theatre is presenting a stage production of the feature "Hoodwinked". At the opening, 70 tickets were sold and ticket sales totaled $568. If adult tickets cost $10 and children's tickets cost $7, how many of each type of ticket were sold?  
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The question doesn't ask for a formula, it asks how many tix were sold.
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a + c = 70 (# of tix)
10a + 7c = 568 (cost of tix)
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Can you finish it?