Let x represent the number of child admission tickets that were sold that day.
Let y represent the number of adult admission tickets that were sold that day.
On Thursday, twice as many adult tickets as child tickets were sold. This means that
y = 2x
Child admission is $5.10 and adult admission is $9.90. Given that the total sales is $697.2, the equation would be
5.1x + 9.9y = 697.2
Substituting y = 2x into the above equation, it becomes
5.1x + 9.9(2x) = 697.2
5.1x + 19.8x = 697.2
24.9x = 697.2
x = 697.2/24.9
x = 28
y = 2x = 2 * 28
y = 56
28 child tickets were sold that day.