A server at a diner serves only burgers and milkshakes to two customers.

• Customer I spends a total of $64 for 6 burgers and 4 milkshakes.

. Customer 2 spends a total of $89 for 8 burgers and 6 milkshakes.

What is the cost of milkshakes?

a. $5.50
b.$6.00
c.$6.40
d.$7.00