Question 1082237
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Your equations are

     c +      a = 136,       (1)
5.30*c + 9.00*a = 913.20.    (2)

From equation (1), express a = 136 - c and substitute it into equation (2) replacing  "a". You will get a single equation for "c":

5.30*c + 9*(136-c) = 913.20.


Simplify and solve for "c", the number of children's tickets.
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This method is called "the Substitution method".