To get the equation we need to resort to the average formula.
The sum of numbers/amount of numbers.
(1445 + 1624 + x) / 3 = 1500
(3069 + x) / 3 = 1500
3069 + x = 4500
x = 1431
So, they would have to earn $1431 on Sunday in order to get the mean average of $1500.