## Exercise 173 Mental Multiplication

Mathematical illiteracy is nothing to brag about with statements like, "I can never balance my checkbook" or "I hate anything to do with math." Indeed, it's a pity when department and grocery store cashiers nowadays have trouble counting out your change if their computer register goes down. Remember, you're never too old to learn speed mathematics. Take the example of Baron Herbert de Grote who was over 60 when he started actively working on mental calculations and at the age of 82 was in the Guiness Book of World Records for his record breaking calculations.

To learn speed multiplication, first practice with adding zeros. For example 800 times 400 has 4 zeros that are simply added to the product of 32 making 320,000. 800 x 400 320,000

Now practice with the following until it becomes automatic: 90 50 300 540 2300 3000 4000 6000 7500 x 30 x 60 x 70 x 80 x 500 x 2000 x 600 x 200 x 400

Even though this seems real easy, it must be mastered and become second nature to you before tackling what comes next. One of the consistent findings that researchers have discovered about lightning calculators is that they multiply from left to right and store only one fact away at a time. This is done in a 'moving' fashion so fast that often calculators cannot explain the procedure until they're given a complex computation that slows them down just enough to let them actually "watch" the mental process in motion. Their conscious self actually watches their subconscious self whiz through the calculation in a time distorted fashion (review "Exercise -- Time Distortion"). The process of lightning calculating has simply become automatic to them.

As with any skill, regular practice brings you proficiency. You can actually learn to creatively have fun playing with numbers. Look for ways to play with numbers every day. For example, multiply how many people go into a building in one minute X 60 to get an hourly traffic guesstimation.

As a right brain faculty, speed multiplication involves a reverse process of calculating from left to right, as opposed to left brain calculating of right to left. You will sometimes find that you can encourage the right brain into performing calculations if you first engage your left brain in a talking or writing mode.

300 x 400 = |
120,000 | |

300 x 60 = |
18,000 |
138,000 |

300 x 2 = |
600 |
138,600 |

400 x 40 = |
16,000 |
154,600 |

400 x 9 = |
3,600 |
158,200 |

40 x 60 = |
2,400 |
160,600 |

40 x 2 = |
80 |
160,680 |

60 x 9 = |
540 |
161,220 |

9 x 2 = |
18 |
161,238 |

When a problem is visibly written, some lightning calculators use criss-cross multiplication. This is a right to left method that can be used in a mental way as well. The problem 349 times 462 becomes:

Units Tens Hundreds Thousands Thousands

Complex Complex Complex Complex Complex

462 8

1238

462 161,238

The steps in criss-cross multiplying are as follows:

63 write 3 5) Carry 4

4 x 9 = 36 and the complete answer is: 161,238 4 x 6 = 24

72 write 2

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