Looking forward to trying this one tomorrow! Looks like a cracker to me to introduce the idea of simple and compound interest in my year 9 maths class…
http://threeacts.mrmeyer.com/frysbank/
Although this one is based on the simple compound interest formula, I think it’s helpful enough to introduce both concepts at the start to help them see a difference.
EDIT: It was great! Kids had all sorts of guesses – the most ‘extreme’ was 1 billion, classic. Wish I captured their responses that they thought were ‘too low’, ‘too high’ or what they actually thought it was. Classic when they saw the answer!
Tomorrow I might reinforce it with this nice YouTube comparison of simple v. compound interest using skittles:
Here is an additional resource that I came across via Twitter thanks to Robert Kaplinsky. I haven’t tried this yet in the classroom but has an Austin Power’s video / guesses etc. too based around compound interest.