## Trick 4: How to be a Mathematical Clairvoyant

June 22, 2018# How to be a Mathematical Clairvoyant

### Requirements: Spooky music and spooky look.

### You will find some spooky music here.

**Tell your students you will beat them adding up 5 x 5 digit numbers in your head when they are using calculators.**

# Method:

### 1. Ask a student to write down 2 x 5-digit numbers on the board.

### 2. You rapidly write a 5-digit number underneath.

### 3. Ask another student to write another 5-digit number.

### 4. You write another 5-digit number quickly.

### 5. You have 5 by 5 digit numbers. Say ‘Go’. You instantly write down the answer.

### Stand back.

# This is how it works:

### N_{1 }= 97413

### N_{2} = 28619

### N_{3} = 71380 (Each digit in N_{3} that you write down must add up to 9 with digits in No. above)

### N_{4} = 64231

### N_{5} = 35768 (Once again each digit in N_{5} must add up to 9 with digits above)

### Now you will instantly write down the sum of these five numbers as

### 297411

### Da! DA!

### The trick is to subtract 2 from N_{1 }and put it in front:

### N_{1 }= 97413

### N_{1 }-2 = 97411

### Sum of 5 numbers = 297411

# This is why it works:

### Hint: When you get your students to add up the five 5-digit numbers on a calculator you will beat them, but they will also get many different answers as a number of students will key incorrect numbers.

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