I'm not entirely sure whether my pupils will have the same perplexity as I do with this. This is from the film 'Swordfish', but I seem to remember this happening in quite a few other films too (contact me if you can name any). The natural question I get is: "What, really? That interest that in my bank account amounts to such a piddling amount can take you from $400 million to $9.5 billion in just 15 years?"

The problem with this might be of course that a lot of my pupils may not have a bank account yet and they aren't maths nerds (yet).

**What I'm going to do:**

**Introduce the session:**

- Show the video.
- Ask what question comes to mind. Do we think their answer is too high/too low? How could we show their answer is too high (upper bound)?
- What information would we need to answer this question (Interest rates since 1986)? Where can we find this information (can lead them here if they can't find one themselves)?

**Main section:**

Pupils should be ok from here if they know how to find the multiplier. Pupils finding 5% first then adding it on will get frustrated at the number of repeated calculations they need to do and will be gagging for an easier way.

**End:**

No way I can see of verifying your answers unfortunately, so we will have

**to compare our answers with each other then share our disappointment with the accuracy of the movie.**

**Extension questions:**

- What interest would we need in order to get $9.5 billion from $400 million in 15 years?
- (Super-extension) How many years would it take if we had a fixed rate of 7% interest?