You look at a clock in a plane mirror, and the image shows the time 2:25. What is the real time?

Step 1. The rule. Reflecting the clock face in the line through $12$ and $6$ turns a time into $12{:}00$ minus that time.

Step 2. Subtract. $12{:}00 - 2{:}25$: write $12{:}00$ as $11{:}60$. Then $11{:}60 - 2{:}25 = 9{:}35$.