20 million people across the U.S. were estimated to have participated in the first Earth Day. More than a billion people in 180 countries around the world now celebrate Earth Day every year. **What is the percentage increase from 20 million to 1 billion?**

Here are some more Earth Day math problems:

- Making new cans from used aluminum cans uses 75 percent less energy than making a can from new materials. It takes 200 million Btu’s (British Thermal Units) to produce a ton of aluminum cans from new materials and only 50 Btu’s to produce a ton of aluminum cans from used cans.
**How many Btu’s are needed to make 3 tons of aluminum cans from used cans? How many cans can be made with 75 Btu’s?** - In 1997, the U.S. recycled 13 million cars, which conserved 32.5 billion pounds of iron ore, 18.2 billion pounds of coal and 1.5 billion pounds of limestone.
**If in 2015 we recycle 18 million cars how much iron ore will be conserved? How much coal will be conserved?** - Recycling 1 aluminum can saves enough energy to run a TV for 3 hours.
**If your TV runs 84 hours this month how many aluminum cans should you recycle to produce the energy needed?** - A leaky faucet can waste up to 20 gallons of water per day, and a leaking toilet can waste up to 200 gallons a day.
**If 3 leaky faucets and 2 leaking toilets were fixed now how many gallons of water would be saved each day?**