Eric and Charles are on their high school track and field team. Eric is participating in hurdles and the javelin. For every 5 hours that Eric spends practicing hurdles, he practices the javelin for 1.5 hours. Charles is participating in a relay race and the javelin. For every 5 hours that Charles spends practicing for the relay race, he practices the javelin for 2.5 hours.
If Eric practiced hurdles for 15 hours last week, and Eric and Charles practiced javelin for the same amount of time, then how long did Charles practice for the relay race last week?
For every 5 hours that Eric spends practicing hurdles, he practices the javelin for 1.5 hours, so for 15 hours of practicing hurdles, he practiced the javelin for 15 / 5 * 1.5 = <<15/5*1.5=4.5>>4.5 hours
If Charles and Eric practiced javelin for the same amount of time, and Eric practiced the javelin for 4.5 hours, then Charles must have practiced the javelin for 4.5 hours as well.
Charles practices the javelin for 2.5 hours for every 5 hours spent practicing the relay race, so for 4.5 hours of practicing the javelin, Charles practiced for 4.5 / 2.5 * 5 = <<4.5/2.5*5=9>>9 hours in the relay race. Answer: \boxed{9}.