ab=240 → b = 240/a (1)
cd=500 → d = 500/c → 500/ (215/a) → 500a / 215 → 100a/43
d-bc=455
ac=215 (4) → c = 215/a
then, d=?
Put (1), (2), (4) into (3)
100a/43 - [ 240/a] * [ 215/a] = 455 multiply hrough by a^2
(100/43)a^3 - 240*215 = 455a^2
(100/43)a^3 - 455a^2 - 51600 = 0
100a^3 - 19565a^2 - 2218800 = 0 → a ≈ 196.23
So....d =100a / 43 = [100*196.23] / 43 ≈ 456.348