31) F

The questions is basically asking us to find out what the smallest amount the person can sell for and largest amount they can sell for.

Firstly we need to find the largest decrease in price and larges increase in price during this 10 day period.

Largest decrease during this 10 day period is a decrease of 10p per share (25 p to 15 p or 30 p to 20 p)

If the person bought at 25p per share and sold at 15p per share:
Number of shares bought at 25 p = 6000 ÷ 0.25 = 24,000 shares
If he sold these shares at 15p per share he would get = 24,000 x 0.15 = £3600

If the person bought at 30p per share and sold at 20p per share:
Number of shares bought at 30 p = 6000 ÷ 0.30 = 20,000 shares
If he sold these shares at 20p per share he would get = 20,000 x 0.20 = £4000

The smaller amount of these is £3600.

The largest increase is an increase of 15p per share from 15p to 30p. There is only one time this happens (day 5 to day 7)

If the person bought at 15p per share and sold at 30p per share:
Number of shares bought at 15p = 6000 ÷ 0.15 = 40,000 shares
If he sold these shares at 30p per share he would get = 40,000 x 0.30 = £12,000
Therefore, the difference between these is = 12000 – 3600 = £8400