Answer: Krista put 0.40 in the first equation meant for the number of stamps
Step-by-step explanation: From the information available to Krista, she can determine the number of stamps that cost 40 cents and those that cost 55 cents. What she needs is a system of simultaneous equations both of which would use the total number of stamps and the total cost of stamps to determine the how many stamps cost 40 cents and how many costs 55 cents.
The system of equations should have been,
x + y = 45 ----------(1)
0.40x + 0.55y = 18.75 ----------(2)
From equation (1), let x be the subject of the equation and hence x = 45 - y
Substitute for the value of x into equation (2)
0.40(45 - y) + 0.55y = 18.75
18 - 0.40y + 0.55y = 18.75
Collect like terms and you now have
0.55y - 0.40y = 18.75 - 18
0.15y = 0.75
Divide both sides of the equation by 0.15
y = 5
When y has been calculated as 5, substitute for the value of y into equation (1)
x + y = 45
x + 5 = 45
x = 45 - 5
x = 40
The results show that the stamps that cost 0.40 cents (x) were 40 in number while those that cost 0.55 cents (y) were 5 in number.
This answer could not have been derived due to the mistake Krista committed when writing the equations.
