It is worth 20p.
20p it's not that old
A pound sterling is divided into 100 pence, so 5 20p pieces are worth £1 (100/20 = 5)
Assuming this is the British 20p, they're all still worth face value.
The answer is 100/(20p) = 5/p
No such coin exists. The 20p coin was introduced during decimalization in the late 1900s, not the late 1800s and the forerunner to the 20p the double florin was discontinued in 1890.