Compounding percentage problems - the 0.7/r approximation

Handy tip for you. If a question has something about X that grows at Y% (assuming Y is fairly small - 1 - 10%), then it takes

0.7/Y% years (or compounding periods) to double.

For e.g. if John earns 5% interest on $1000, it will take ~ 0.7/0.05 = 70/5 =~ 14 years for that money to double.

This can be very handy especially when you face questions on company growths, investments etc. where some sum of money is growing at some rate for X years and you need to figure out what it is in N years. By using this approximation, you can quickly figure out when it would double.