27) B

- Number of plates of prawns that can be prepared in a 400 g pack = 400 ÷ 50 = 8
- Number of plates of cockles that can be prepared in 200 g packs = 200 ÷ 50 = 4
- Number of plates of whelks that can be prepared in 200 g packs = 200 ÷ 50 = 4
- Number of plates of salmon that can be prepared in 200 g packs = 200 ÷ 50 = 4
- Number of plates of squid that can be prepared in 200 g packs = 300 ÷ 50 = 6

We must find the lowest common multiple of 8, 4 and 6. This is 24.

So we must prepare 24 plates. This will allow no seafood to be left over.

- Number of prawn packs needed to make 24 plates = 24 ÷ 8 = 3 packs. If Charlie buys 2 packs, she gets third free. So total cost of prawns = €4.08 x 2 = €8.16
- Number of cockles packs needed to make 24 plates = 24 ÷ 4 = 6 packs. If Charlie buys 4 packs, she gets two packs free. So total cost of cockles = €4.08 x 4 = €16.32
- Number of whelks packs needed to make 24 plates = 24 ÷ 4 = 6 packs. If Charlie buys 4 packs, she gets two packs free. So total cost of whelks = €4.08 x 4 = €16.32
- Number of salmon packs needed to make 24 plates = 24 ÷ 4 = 6 packs. If Charlie buys 4 packs, she gets two packs free. So total cost of salmon = €4.08 x 4 = €16.32
- Number of squid packs needed to make 24 plates = 24 ÷ 6 = 4 packs. When Charlie buys the first two packs, she gets the third free. However she will have to pay for the fourth pack. In total, she will have to pay for 3 packs. So total cost of squid = €4.08 x 3 = €12.24

Total amount spent = 8.16 + 16.32 + 16.32 + 16.32 + 12.24 = €69.36

Cost of each plate = 69.36 ÷ 24 = €2.89