Cost of each plushie is twice the cost of each keychain.

2 keychains = 1 plushie

4 keychains = 2 plushies

4 keychains + 8 plushies = 10 plushies

1/4 ( = 2/8) of his money bought him 10 plushies

Remaining money = 8/8 – 2/8 = 6/8

1/2 of remaining money = 1/2 x 6/8 = 3/8

2/8 can buy 10 plushies

1/8 can buy 5

3/8 can buy 15

Mr Robins bought a total of (8 + 15) = 23 plushies

You were almost there.

On the last step, you added 10p instead of 8p.

Remember that he didn’t actually buy 10 plushies at first. He bought 8 plushies + 4 keychains, which you equated to have the same cost as 10 plushies since cost of 1 plushie = cost of 2 keychains.

Thank u for the explanation . It’s clear now