At the city museum, child admission is $5.20 and adult admission is $8.30. On Monday, three times as many adult tickets as child tickets were sold, for a total sales of $1173.90

Let # of child tickets = c

# of adult tickets = 3c

3c (8.30) + c (5.20) = 1173.90

24.9c + 5.2c = 1173.90

30.1c=1173.90

c=39 tickets

3c=39x3 = 117 tickets

Therefore, 39 child tickets and 117 adult tickets were sold on monday, thus, a total of 156 tickets were sold.