Magic Box
Grade: 3-8
Your Grandma gives you a wooden box for your birthday. When you unwrapped the gift and opened the box, you were expecting some cash inside but found the box was empty.
You felt bad for not appreciating the box, and decided you could use it to store important things. Just before you go to bed, you open the box to put a photo inside, and see 2 pennies.
When you check the box at the end of the second day, you count 4 pennies.
At the end of Day 3, you count 8 pennies.
How many pennies will be in the box after 10 days? 15 days? 20 days? Can you come up with a rule to find the number of pennies in the box on any day?