Kirsten and Molly are on a 3-day bike ride. The entire route is 156 miles. On the first day they biked 64 miles, and on the second day they biked 45 miles. How many miles do they have left to bike on the third day?

The distance that Kirsten and Molly need to bike on their third day is obtained by subtracting the sum of the distances they have traveled after two days from the length of the entire route.

distance left = 156 miles - (64 miles + 45 miles)

= 47 miles

Thus, Kirsten and Molly need to bike 47 miles on their third day.