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?