Start with the highest number of the luxury gift tins with a total cost below £100:
19 * £5 = £95
Assume lowest number of normal biscuits:
1 * £1 = £1
Calculate the number of minipacks needed to make up the difference:
40 * 10p = £4
However, the total number of items is 60. Therefore, progressively reduce the number of luxury gift tins by 1, increase the number of normal biscuits by 1, and make up the difference in minibiscuits:
18 *£5 = £90
2 * £1 = £2
80 * 20p = £8
This adds up to the target total of 100 items, so this is the answer