Answer:
Option A.
Explanation:
there were 45 stamps purchased for $18.75. Some stamps were 40 cents, and some stamps were 55 cents.
Let x and y be the number of stamps of 40 cents and 55 cents respectively.
Total number of stamps is 45. So
Total cost of stamps is $18.75. So
But Krista wrote the system of equations as
Krista put 0.40 in the first equation meant for the number of stamps.
Krista did not write 0.40 in the second equation meant for the value of stamps.
Therefore, the correct option is A.