Hint: The fraction of the time you spend in some activity is equal to the fraction of people doing it right now!
How long do you estimate a person spends doing this activity? About one minute seems a reasonable estimate.
That makes seven billion of us spend an average of 1 minute per day picking our nose (EW!)
There are about 25 * 60 = 1500 minutes per day. (We use 25 and not 24 as it is easier to work with nice, round numbers)
Or, the number of people picking their nose right now will be
7.8 * 10^9 billion * 1 / 1500 minutes
Or, about 5,200,000 people (or about 5 million people)