At the city museum, child admission is $5.40 and adult admission is $8.70 . on friday, 170 tickets were sold for a total sales of $1224.90 . how many adult tickets were sold that day?

X=number of adult tickets
y=number of children tickets.
we can suggest this system of equations:

x+y=170
8.7x+5.4y=1224.90

We can solve this system of equations by substitution method:
x=170-y

8.7(170 - y)+5.4 y=1224.90
1479-8.7y+5.4y=1224.90
-3.3 y=1224.90-1479
-3.3 y=-254.1
y=(-254.1) /(-3.3)
y=77

x=170-y
x=170-77
x=93

93 adult tickts were sold that day.

Yes, two vertical lines can be complementary