what is eleven fourty plus thirty minutes
11:40 is 20 minutes away from 12:00, so that plus 30-10 is 12:00. Add additional 10 minutes and we have 12:10.
If you don't understand anything feel free to ask.
Add 30 minutes to 11:40 and get 11:70. But the clock doesn't go to 70, that's 10 minutes too far, so the time is 12:10.
This is a cinch to visualize if you're familar with analog clocks, i.e., the kind with an hour hand and a minute hand.