G3.4 Three prime numbers $d, e$ and $f$ which are all less than 10 , satisfy the two conditions $d+e=f$ and $d<e$. Find $d$.
Possible prime numbers are $2,3,5,7$.
$$
\begin{array}{l}
2+3=5 \text { or } 2+5=7 \\
\therefore d=2
\end{array}
$$