We need to work out the average time that Bob would take to deal with a questionnaire.

80% of them are short and he takes 10 minutes, so we do 80%/100% * 10 minutes = 8 minutes.

The other 20% are long and take 30 minutes, so we do 20%/100% * 30 minutes = 6 minutes.

Add these to get the total average time and we get 14 minutes.

He works for 7 hours, which is 420 minutes.

Therefore, he can deal with 420/14 = 30 questionnaires in 1 day.