Answer: 57 child tickets were sold that day.
Explanation:
Let x represent the number of child tickets that were sold.
Let y represent the number of adult tickets that were sold.
on Friday, 155 tickets were sold. It means that
x + y = 155
At the city museum, child admission is $5.70 and adult admission is $9.50. The ticket sales totaled $1255.90 on that day. It means that
5.7x + 9.5y = 1255.9- - - - - - - - - 1
Substituting x = 155 - y into equation 1, it becomes
5.7(155 - y) + 9.5y = 1255.9
883.5 - 5.7y + 9.5y = 1255.9
- 5.7y + 9.5y = 1255.9 - 883.5
3.8y = 372.4
y = 372.4/3.8
y = 98
x = 155 - y = 155 - 98
x = 57