Question

suppose stamps for postcards cost $0.28 and stamp for first class letter cost $0.44 diego wanted to send postcards and and letter to 10 friends if he had 3.60 for stamps and spends all of it how many postcards and how many letters could he send

Answer 1

He could buy 12 postcards for $3.36.
and 8 letters for $3.36

Answer 2

0.28 - Stamp
0.44 - Letter
0.72 - Both Letter and stamp 
If we subtract 0.72 until we get 0 or no longer can subtract it from the 3.60 

 3.60
-0.72
---------
 2.88
-0.72
---------
 2.16
-0.72
--------
 1.44
-0.72
--------
 0.72
-0.72
--------
000

So he can send only 5 postcards and letters
(This is how I understood this. Correct me if I am wrong.)