The school hiking club has completed 4 out of 5 hikes so far this year. They have hiked 4.6 miles, 3.7 miles, 5.1 miles, and 2.9 miles. If their goal is to hike 20 total miles, how many miles does the last hike need to be?
a.4.7 miles
b.3.7 miles
c.2.7 miles
d.1.7 miles