The thermal efficiency is defined as follows
\(\eta = 1 - \frac{Q_{\text{out}}}{Q_\text{in}}\),
and the energy which is put into the system is
\(Q_{\text{in}} = W_\text{out} + Q_\text{out}\).
In your case \(Q_\text{in} = 25 \text{ J},W_\text{out}=10 \text{ J}.\)
So \(Q_\text{out}=10 \text{ J}\) which gives an efficiency of
\(\eta = 1 - \frac{10 \text{ J}}{25 \text { J}} = 0.6 = 60 \%\).
