Excuse me! i´m venezuelan. i´m a spanish teacher, but sometimes i teach english to some students. the question is : according to the compound adjective rules, which of these statements is right? "a 15-page book" or "a 15-paged book", "a four-letter word" or "a four-lettered word".
i´ll your answers!
a doubtful venezuelan teacher i

Ending the adjectives with -ed is not needed, so those answers are out. As for the numbers, the rule is usually that numbers one through nine are spelled out whereas number 10 and up are written in numbers.  That said, you do not hyphenate the number with the word, so four-letter word would be correct.

b. his ability to use formal language to stress strong feelings

in my opinion, ap calculus was the most difficult. i often struggled with it a lot. however, i have gotten a lot better.