p + r = 18
35p + 49r = 728
solve the system by substitution or elimination
35p + 49r = 728
solve the system by substitution or elimination
From the given information, we know that the cost of postcard stamps is $0.35 and regular mail stamps are $0.49. We also know that Amy bought a total of 18 stamps and spent a total of $7.28.
Using this information, we can set up the following system of equations:
Equation 1: x + y = 18 (total number of stamps bought)
Equation 2: 0.35x + 0.49y = 7.28 (total cost of stamps)
Now, let's solve this system of equations to find the values of x and y.
First, let's multiply Equation 1 by 0.35 (the cost of postcard stamps) to eliminate x from Equation 2:
0.35x + 0.35y = 6.3
Now, subtract this equation from Equation 2 to eliminate x:
(0.35x + 0.49y) - (0.35x + 0.35y) = 7.28 - 6.3
0.49y - 0.35y = 0.98
0.14y = 0.98
Dividing both sides by 0.14, we get:
y = 7
Now, substitute the value of y back into Equation 1 to find x:
x + 7 = 18
Subtracting 7 from both sides, we get:
x = 11
Therefore, Amy bought 11 stamps for postcards and 7 stamps for regular mail.