Can someone help me on this? IN NEED OF HELP!

Two customers went to a post office. first customer paid $12 for 14 and 5 envelopes the second paid 24.80 for 10 postcards and 15 envelopes. what is cost of each envelope?

Given a: b and b: c, find a: b: c. write the ratio in simplest form. a: b=6: 10 and b: c=21: 33

Which steps could be part of the process in algebraically solving the system of equations, y+5x=x^2+10 and y=4x-10