Suppose stamps for postcards cost $0.28 and stamps for first-class letters cost $0.44. Diego wanted to send postcards and letters to 10 friends. If he had $3.60 for stamps and spends it all, how many postcards and how many letters could he send?

5; 5

Explanation:

Let us assume that the postcard be X

And the letters be Y

So there are two equations which are presented below:

X + Y = 10 - equation 1

$0.28X + $0.44Y = $3.60 - equation 2

Now we simplify the equation 2 by multiplying with the 100 and dividing by 4

7X + 11Y = $90

Now we substitution the both equations

X + Y = 10

7X + 11Y = $90

Multiplied by 7 in first equation

So,

7X + 7Y = 70

7X + 11Y = $90

4Y = 20

Y = 5

As we know

X + Y = 10

So, X would be

= 10 - 5

= 5