Krista was assigned a homework problem that stated there were 45 stamps purchased for $18.75. Some stamps were 40 cents, and some stamps were 55 cents. To solve this problem, she wrote the system of equations that is shown below. 0.40 x + y = 45. x + 0.55 y = 18.75.

Which explains the error that Krista made?
Krista put 0.40 in the first equation meant for the number of stamps.
Krista put 0.55 in the second equation meant for the value of stamps.
Krista did not use the correct decimal to represent the total cost of the stamps.
Krista mistakenly put 45 in the first equation when it should have been in the second equation.