Question 1123309
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The condition says us that


<U>Adult tickets</U> + <U>student tickets</U> = 315,   or


A             + S               = 315.    (A stays for adults tickets, while S stays for the student ones).


It also says that


S = 2A.


So you can replace S in the very first equation by 2A. Then you get


A + 2A = 315,   or

3A = 315,


which implies  A = 315/3 = 105.


<U>Answer</U>.  105 adult tickets were sold.
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Solved.