For this case we have:
m represents the total cost of each meal. If people do not pay a tip, the total cost to pay would be:

Person 1:
Without tip the cost of your meal is m.
You want to pay 10% tip
m -------------------> 100%
x --------------------> 10%

Thus, its cost will be:

Person 2:
Without tip the cost of your meal is m.
m -------------------> 100%
x --------------------> 15%

Thus, its cost will be:

Person 3:
Without tip the cost of your meal is m.
m -------------------> 100%
x --------------------> 20%

Thus, its cost will be:

Thus, the total cost to pay will be:

Answer:
