Percentage = attended ÷ held × 100
Classes needed for 75%: attending x more classes gives (attended + x) ÷ (held + x) = 0.75, so x = (0.75 × held − attended) ÷ 0.25, rounded up.
Safe bunks: skipping y classes keeps attended ÷ (held + y) ≥ 0.75, so y ≤ (attended − 0.75 × held) ÷ 0.75, rounded down.
Both assume you attend (or skip) every class from now on — one bunk resets the "needed" count.