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…
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
Trick No. 2 – Dividing with Zeros
Numbers ending in zeros are just multiples of ten (or one hundred, or one thousand…) To divide with a number ending in zeros, ignore the zeros, do your division, then cancel the same number of zeros from the left and the right of the problem*
Trick No. 1 – Multiplying with Zeros
Numbers ending in zeros are just multiples of ten (or one hundred, or one thousand…) To multiply by a number ending in zeros, just ignore the zeros, do your multiplication, then add the zeros back on!