Question

What is the formula of the ethyl group?

Answer 1

Ethyl is derived from ethane `C_2H_6` or better `H_3C-CH_3`

If one of the `H`'s is removed, it becomes a one-bond group, that can attach to a carbon-atom in stead of a `H-`

Ethyl is thus `C_2H_5-`
(the dash at the end represents the 'open' bond)

Example
If one of the `H`'s of benzene `C_6H_6` is replaced by an ethyl-group, you get ethyl-benzene `C_2H_5-C_6H_5`

(pictures from Wikipedia)
The ethyl-group is at the top of the benzene-ring.