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