At the city museum, child admission is $5.90 and adult admission is $9.60 . on friday, 170 tickets were sold for a total sales of $1339.70 . how many child tickets were sold that day?

This is the concept of simultaneous equations, let the number of child tickets sold be x and that of adult be y;
thus:
x+y=170
hence:
x=170-y
also:
5.9x+9.6y=1339.70
substituting the value of x and solving for y in the above equation we get:
5.9(170-y)+9.6y=1339.7
1003-5.9y+9.6y=1339.7
-5.9y+9.6y=1339.7-1003
3.7y=336.7
y=336.7/3.7
y=91
x=170-91
x=79
thus the number of child tickets sold was 79 tickets