Jeremy worked for 8 1/2 hours building model airplanes. If each airplane takes him 1 2/5 hours to make, how many models did he build?
First, we need to convert the time Jeremy worked into a mixed number. We know that 1 hour is equal to 1 whole and 0/5 parts of an hour, so 8 1/2 hours is equal to 8 + 1/2 = 8 2/10 hours.
Next, we need to convert the time it takes Jeremy to build one airplane into a mixed number. 1 2/5 is 1 + 2/5 = 1 4/10 hours.
Now we can divide the total time Jeremy worked (8 2/10 hours) by the time it takes him to build one airplane (1 4/10 hours) to find the number of airplanes he built: 8 2/10 hours / 1 4/10 hours/airplane = 8.2 / 1.4 = 5.878787878787879
Jeremy built 5 models. Answer: \boxed{5}.