Answer:She will have to contact a 100 persons.
Explanation:
Working backwards, 5% of the potential 20% of donors would give more than or equal to a $100. So,
The probability of a donor giving a $100 or more = 5% * 20%
=5/100 * 20/100 = 0.01 =1%
So on average 1% of the total population called would be $100 donor. So she would need to call a 100 persons on average to get that 1 person.