3. (5 points) The average of four numbers is 30. If one of them is changed to 50, the average becomes 40. The original number was $\qquad$
【Solution】Solve: $50-(40 \times 4-30 \times 4)$
$$
\begin{array}{l}
=50-(160-120) \\
=50-40 \\
=10
\end{array}
$$

Answer: The original number is 10.
Therefore, the answer is: 10.