With lead removed from car emissions, along came MTBE.

Methyl tert-butyl ether

(CH₃)₃COCH₃

tert or tertiary or of the third order

CH3 is methyl. One carbon and three hydrogen.

(CH3)3 - methyl molecules connected to a common carbon

butyl is when there are four carbons. In this case it is C4H9. 

Butyl is derived from butane. Butane has a formula of C4H10.

The molecule C4H10 was broken down to form MTBE. The four carbons remained hence "butyl."'

Ether is any of a class of organic compounds that has an oxygen atom attach to two carbon atoms.

The diagram of MTBE has one oxygen atom. It is the red one. It is between two carbons. 

C0CH3 is the other half of MTBE, hence the word ether in the name.

A total of five carbons in this molecule.