Multiplying two 2-digit- Perkalian 2 Digit

Multiplying two 2-digit numbers(same 1st digit)

1. Select two 2-digit numbers with the same first digit.
2. Multiply their second digits (keep the carry). _ _ _ X
3. Multiply the sum of the second digits by the first digit,
add the carry (keep the carry). _ _ X _
4. Multiply the first digits (add the carry). X X _ _

Example:

1. If the first number is 42, choose 45 as the second number (any 2-digit number with first digit 4).
2. Multiply the last digits: 2 × 5 = 10 (keep carry)
_ _ _ 0
3. Multiply the sum of the 2nd digits by the first:
5 + 2 = 7; 7 × 4 = 28; 28 + 1 = 29 (keep carry)
_ _ 9 _
4. Multiply the first digits (add the carry)
4 × 4 = 16; 16 + 2 = 18
1 8 _ _

5. So 42 × 45 = 1890.

See the pattern?

1. If the first number is 62, choose 67 as the second number
(any 2-digit number with first digit 6).
2. Multiply the last digits: 2 × 7 = 14 (keep carry)
_ _ _ 4
3. Multiply the sum of the 2nd digits by the first (add carry):
2 + 7 = 9; 6 × 9 = 54; 54 + 1 = 55 (keep carry)
_ _ 5 _
4. Multiply the first digits (add the carry)
6 × 6 = 36; 36 + 5 = 41
4 1 _ _

5. So 62 × 67 = 4154.

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Mathematics Trick

1. Multiplication by 5
It's often more convenient instead of multiplying by 5 to multiply first by 10 and then divide by 2.
For example,
137·5 = 1370/2 = 685.

2. Division by 5
Similarly, it's often more convenient instead to multiply first by 2 and then divide by 10.
For example,
1375/5 = 2750/10 = 275.

3.Division/multiplication by 4
Replace either with a repeated operation by 2.
For example,
124/4 = 62/2 = 31. Also,
124·4 = 248·2 = 496.

4.Division/multiplication by 25
Use operations with 4 instead.
For example,
37·25 = 3700/4 = 1850/2 = 925.

5.Division/multiplication by 8
Replace either with a repeated operation by 2.
For example,
124·8 = 248·4 = 496·2 = 992.

6.Division/multiplication by 125
Use operations with 8 instead.
For example,
37·125 = 37000/8 = 18500/4 = 9250/2 = 4625.

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