Jason, this is the solution:
Walter's daily wage = $ 50
Tips = 15% of the total cost of the dinner he serves
Tuesday earnings = $ 170
Therefore,
Tips = 170 - 50
Tips = 120
For finding the cost of the dinners, we use Direct Rule of Three, as follows:
Percentage Cost
15 120
100 x
_____________________
120 * 100 = 15 * x
12,000 = 15x
Dividing by 15 at both sides:
15x/15 = 12,000/15
x = 800
The total cost of the dinners Walter serverd on Tuesday was $ 800