When a mathematician writes qed at the end of a proof, what does this mean?

Write the sentence as an equation. 6 fewer than the quantity 140 times j equals 104

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?

What is the answer to this question? i'm a bit stumped. also how do i do it? 5(x - 4) = 2(x + 5)