Recall that the two acute angles of a right triangle add up to 90 degrees.
Let s be the measure (in degrees) of the smallest angle of the given right triangle, then we can set the following equation:
![s+40+s=90.](https://img.qammunity.org/2023/formulas/mathematics/college/hwlildecm864exzyut6l6pcdwjg93i5wek.png)
Adding like terms we get:
![2s+40=90.](https://img.qammunity.org/2023/formulas/mathematics/college/n5yftqf989o3fuwz13yodqta0oehsl8okm.png)
Subtracting 40 from the above result we get:
![\begin{gathered} 2s+40-40=90-40, \\ 2s=50. \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/3xz3xvd141glgusknuva37o1e1pfd91v9a.png)
Dividing the above result by 2 we get:
![\begin{gathered} (2s)/(2)=(50)/(2), \\ s=25. \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/1c5jka23j7c51x794vs67qwscyn8r6jqwq.png)
Therefore the measure of the smallest angle is 25º.
Answer: 25º.