A 2-column table with 5 rows. Column 1 is labeled Number of friends helping with entries 0, 1, 2, 3, 4. Column 2 is labeled Hours to build tree house with entries 20, 20 minus 20 (0.10) = 18.0, 18 minus 18 (0.10) = 16.2, 16.2 minus 16.2 (0.10) = 14.6, question mark. Peter figures he can build the entire tree house by himself in 20 hours. He figures that each additional friend who helps him would improve the time by 10%. He created a table using the algebraic expression n – n(0.10), where n is the previous number of hours.

To the nearest tenth of an hour, how long will it take to build the tree house if 4 friends are helping?