At the city museum, child admission is $5.30 and adult admission is $9.40 . on tuesday, 140 tickets were sold for a total sales of $1086.40 . how many child tickets were sold that day?

Total tickets sold = # of kid's tickets sold + # of adult tickets sold
Total money made = amount of money kid's tickets raised + amount the adult tickets made

These can be rephrased as
140 = k + a
and
$1086.40 = $5.30k + $9.40a

Rephrase the first;
140 = k + a
a = 140 - k

Plug the equation into the first;
1086.4 = 5.3k + 9.4a
1086.4 = 5.3k + 9.4(140 - k)
1086.4 = 5.3k + 1316 - 9.4k
-229.6 = -4.1k
k = 56 tickets

I would think it is 86 im not sure if i did the math correctly let me go check hold on