Answer:6 postcards and 9 letters
Step-by-step explanation:
First we label what we are looking for
Label number of postcard sent as x
Label number of letter sent as y
The we know Mary wrote to 15people, either as postcard or letter, so from this we make an equation
x + y = 15 (1)
The we know the amount Mary used to send all this is $6.15 which is also 615 cents
Then we know sending one postcard is 35cents, and we labeled number of postcards to be x
So total amount used for postcard will be 35x
And we know sending one letter is 45 cents, and we labeled number of letters to be y
So total amount used for letter will be 45y, so adding all this will be equals to the total amount used which is given as $6.15 also 615 cents, so making another equation
35x + 45y = 615 (2)
Solving the two simultaneously equations
x + y = 15 (1)
35x + 45y = 615 (2)
So we make y the subject of formula in (1)
y = 15 - x (3)
So we put (3) in (2)
Anywhere we see y, we put 15-x
35x + 45(15-x) = 615
35x + 675 -45x = 615
35x - 45x = 615 - 675
-10x = -60
divide both sides by -10
x = 6
So therefore, number of postcards is 6
Then we put x = 6 in (3)
y = 15 - x
y = 15 - 6
y = 9
So therefore number of letters is 9
