trish sends letters ans postcard to her friends.She needs 37 cents in postage for each letter and 21 cents for each postcard. She spends$3.11 in postage. How many letters and postcards did trisha send in all?

Answer 1

you want whole number solutions, ordered pairs, for the equation

37L + 21P = 311

This is an example of a Diophantine's equation.
Somehow I don't think you are studying that topic
by just doing simple trial and error, I got
L = 5, and P=6

check: 5x37 + 6x21 = 311