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.

Example 1: 32 x 4
Step 1: Double the 32: 32 x 2 = 64
Step 2: Double the 64: 64 x 2 = 128 (the answer)

Example 2: 18 x 4
Step 1: Double the 18: 18 x 2 = 36
Step 2: Double the 36: 36 x 2 = 72 (the answer)

Example 3: 2.4 x 40
Step 1: Disregard the decimal point and zero and think “24 x 4″
Step 2: Double the 24: 24 x 2 = 48
Step 3: Double the 48: 48 x 2 = 96
Step 4: Apply ToR (Test of Reasonableness): The original problem was a little more than 2 times 40, which would be more than 80. The final answer should therefore not be 9.6 or 960, but 96.

Example 4: 1900 x 0.4
Step 1: Disregard the decimal point and zero and think “19 x 4″
Step 2: Double the 19: 19 x 2 = 38
Step 3: Double the 38: 38 x 2 = 76
Step 4: Apply ToR (Test of Reasonableness): The original problem was a little more than 1900 times 0.4, which is close to 2000 times about a half (.5 would be a half). 2000 times 0.5 would be 1000, so the final answer should be close to that. Our intermediate answer was 76, so we add enough zeros to bring the answer close to 1000. 76 isn’t big enough; 7600 is too big. 760 is close to 1000. The final answer is therefore 760.