At the movie theatre, child admission is $5.20 and adult admission is $8.40. On Sunday, 170 tickets were sold for a total sales of $1172.00. How many child tickets were sold that day?

Total # of tickets = # of kids + # of adults, or

170 = k + a

total money made = price of a kid's ticket*# of kids tickets + price of an adults ticket*# of adult tickets, or

$1172.00 = $5.20k + $8.40a

Rewrite the first equation.

170 = k + a

a = 170 - k

Plug the first equation into the second.

1172 = 5.2k + 8.4 (170 - k)

1172 = 5.2k + 1428 - 8.4k

-256 = - 3.2k

k = 80 tickets