F.4 Maths

設2年前車子的價值是x﹐則現在的價值是0.68x

據題意0.68x = x * r% * (r - 5)%

0.68 = r(r - 5)/100^2

6800 = r(r - 5)

r = 85

let P be the price 2 years ago
last year: L = P(r%)
now: N = L(r-5)% = P(r%)(r-5)% = P(68%)
(r%)(r-5)% = 68%
(r/100) [(r-5)/100] = 68/100
r [(r-5)/100] = 68
r^2 - 5r = 6800
r^2 - 5r - 6800 = 0
(r-85)(r+80) = 0
r = 85 or -80 (neglected)