problem: Six construction workers that work at the same rate can build a house in 1.5 days.  How long does it take 15 construction workers, working at the same rate, to finish building a house?  Express your answer as a fraction in lowest terms.
solution: The number of workers is inversely proportional to the amount of time it takes to build a house.  Thus $$\text{number of workers} \times \text{time} = \text{constant}.$$Using this, we have that $6 \cdot 1.5 = 15 \cdot t \Rightarrow t = .6$.  As a fraction in lowest terms this is $\boxed{\frac{3}{5}}$.