Imagine you transfer heat into a gas. This heat splits up in two different ways: Additional thermal energy making the gas hotter and "fuel" for the gas to do work. Let's suppose, 64% of the transferred heat transforms into thermal energy. How many percent of the initial heat can be used to do work?

36% of the initial heat can be used to do work.

Explanation:

The initial heat is the 100%, i. e. it is all the heat that will be transferred.

It wil be transferred into two different kinds of energy:

additional thermal energy that will increase the thermal energy of the gas, making it hotter. Call it U. U = 64%. And,

"fuel" for the gas to do work. Call it W.

Then,

100% = U + W W = 100% - U W = 100% - 64% = 36%

That is, 36% of the initial heat is available to do work.