Answer:
310 minutes
Explanation:
1. Find the minutes Marta exercises in a month that has 31 days if she exercises 30 minutes each day:

Martha exercises 930 minutes in a month.
2. Find the minutres Greg exercises in a month that has 31 days if he exercises 40 minutes each day:

Greg exercises 1240 minutes in a month.
3. Calculate how many more minutes Greg does exercise than Marta:
1240 - 930 = 310 minutes