Trick No. 6 – Rapidly Multiply by 4*
* or 0.4, 40, 400, etc.
To multiply a number by four, double it and then double it again! Remember to disregard any decimal points or zeros when starting the calculation.
Trick No. 5 – Multiplying and Dividing with Decimal Points
This trick actually incorporates a few other tricks. Some we’ve already learned. One in particular is brand new, and another isn’t so much a trick as a sensible approach…
The Learning Triangle
I’ve been reading What Counts? by Brian Butterworth. It’s a very worthwhile read for anyone interested in the psychology of learning. The book’s subtitle “How Every Brain is Hardwired for Math*” is appropriate, but not quite complete. I think Mr Butterworth’s book, although aimed at understanding the mathematical brain, goes a long way to explain [...]
Trick No. 4 – Squaring a Number
I read this trick in a book called What Counts? by Brian Butterworth.
To square a number, say “N”, add a small number “a” to it to make it a round number.
Then find (N+a) x (N-a) +a².
Here are some examples:
Trick No. 3 – Multiplying Positive and Negative Numbers
This is a great mnemonic (way of remembering) what the sign of your answer should be when multiplying positive and negative numbers:
Think:
good = pos
bad = neg