Volume · Challenge problem

A shipping box measures $30$ cm by $20$ cm by $15$ cm on the inside. A gift box $20$ cm by $12$ cm by $10$ cm is put inside it, and all the rest of the space is filled with packing peanuts. Packing peanuts are sold in bags of $1.5$ liters. How many bags must be bought? ($1$ liter $= 1000$ cubic cm.)