F1 Maths Questions 快快快~

Question
In a book shop, the initial profit made by selling a calendar was $18. After 2/5 of the calendars had been sold, the bookseller reduced the price of each calendar by $10. It is given that if all the calendars sold, the bookseller made an overall profit of $3000.

(a) Find the total number of calendars he had originally.

(b) If only a further 100 calendars were sold after the price reduction and the rest were discarded, and the bookseller made an overall profit per cent of 20%, find the total cost of the calendars.

Solution
(a)
Let x be the total number of calendars he had originally.
18 * (2/5)x + (18 - 10) * (1 - 2/5)x = 3000
18 * 0.4x + 8 * 0.6x = 3000
7.2x + 4.8x = 3000
7.2x + 4.8x = 3000
12x = 3000
x = 250
The total number of calendars he had originally is 250.

(b)
The profit is
$18 * (2/5)(250) + $(18 - 10) * (100)
= $2600

Profit = Cost * (Profit %)
$2600 = Cost * 20%

Cost = $2600 ÷ 0.2 = $13000

(a)
Let x be the total number of calendars he had originally.
18 * (2/5)x + (18 - 10) * (1 - 2/5)x = 3000
18 * 0.4x + 8 * 0.6x = 3000
7.2x + 4.8x = 3000
7.2x + 4.8x = 3000
12x = 3000
12x/12=3000/12
x = 250
The total number of calendars he had originally is 250.

(b)
The profit is
18 * (2/5)(250) + $(18 - 10) * (100)
= $2600

Profit = Cost * (Profit %)
$2600 = Cost * 20%

Cost = $2600 / 0.2 = $13000