唔識做功課67

14.
(a)
10 × (1 + p%) × [1 ‒ (p ‒ 10)%] = 9
10 × (1 + 0.01p) × (1.1 ‒ 0.01p) = 9
‒0.001p² + 0.01p + 11 = 9
p² ‒ 10p ‒ 2000 = 0
(p ‒ 50)(p + 40) = 0
p = 50 or p = ‒40 (rejected)

(b)
H = 10 × (1 + 50%)
H = 15

Time taken when the stock price was increasing
= (15 ‒ 10) / 0.25 min
= 20 min

Time taken when the stock price was decreasing
= (60 ‒ 20) min
= 40 min

Rate of decreased
= $(15 ‒ 9)/(40 min)
= $0.15/min

(c)
The highest stock price was at 10:20.
10:35 is 15 minutes after the time of the highest stock price.

The stock price at 10:35
= $(15 ‒ 0.15 × 15)
= $12.75
≠ $13

Johnny does NOT tell the truth.