Answer:
Grace sold 16 tickets.
Explanation:
We have a total of 71 tickets sold between 3 people. Blake sold 27 tickets and Chang sold 28 tickets. Adding those sold by Blake and those sold by Chang, gives us: 27 + 28 = 55, if that amount is subtracted from the total sales that are 71 tickets, the result would be: 71 - 55 = 16, which would be the tickets sold by Grace.