Hey there,
To do this you need to work bout what 10% of 340 minutes is.
To do this do 340 / 10 = 34
34 is 10% of 340.
Now you should subtract the 10% from the original time.
340 - 34 = 306.
The answer is 306 minutes.
That was the long method. The short method is to use percentage multipliers.
To do this you do 340 x 0.9 = 306.
Either way you would get the same answer of 306 minutes.
Hope This Helps You!
Good Luck Studying :)