Divisibility by 16
A number is divisible by 16 if the number formed by the last four digits is divisible by 16.
Example1: Check if 5696512 is divisible by 16.
The number formed by the last four digits of 5696512 = 6512
6512 is divisible by 16. Hence 5696512 is also divisible by 16.
Example2: Check if 3326976 is divisible by 16.
The number formed by the last four digits of 3326976 = 6976
6976 is divisible by 16. Hence 3326976 is also divisible by 16.
Example3: Check if 732374360 is divisible by 16.
The number formed by the last three digits of 732374360 = 4360
4360 is not divisible by 16. Hence 732374360 is also not divisible by 16.
Divisibility by 17
To find out if a number is divisible by 17, multiply the last digit by 5 and subtract it from the number formed by the remaining digits.Repeat this process until you arrive at a smaller number whose divisibility you know.If this smaller number is divisible by 17, the original number is also divisible by 17.
Example1: Check if 500327 is divisible by 17.
Given Number = 500327
50032 – (7 × 5 )= 50032 – 35 = 49997
4999 – (7 × 5 ) = 4999 – 35 = 4964
496 – (4 × 5 ) = 496 – 20 = 476
47 – (6 × 5 ) = 47 – 30 = 17
17 is divisible by 17. Hence 500327 is also divisible by 17
Example2: Check if 521461 is divisible by 17.
Given Number = 521461
52146 – (1 × 5 )= 52146 -5 = 52141
5214 – (1 × 5 ) = 5214 – 5 = 5209
520 – (9 × 5 ) = 520 – 45 = 475
47 – (5 × 5 ) = 47 – 25 = 22
22 is not divisible by 17. Hence 521461 is not divisible by 17
Divisibility by 18
A number is divisible by 18 if it is divisible by both 2 and 9.
Example1: Check if 31104 is divisible by 18.
31104 is divisible by 2. 31104 is also divisible by 9. (Please check the divisibility rule of 2 and 9 to find out this)
Hence 31104 is divisible by 18
Example2: Check if 1170 is divisible by 18.
1170 is divisible by 2. 1170 is also divisible by 9. (Please check the divisibility rule of 2 and 9 to find out this)
Hence 1170 is divisible by 18
Example3: Check if 1182 is divisible by 18.
1182 is divisible by 2 , but 1182 is not divisible by 9. (Please check thedivisibility rule of 2 and 9 to find out this)
Hence 1182 is not divisible by 18
Example4: Check if 1287 is divisible by 18.
1287 is not divisible by 2 though it is divisible by 9. (Please check the divisibility rule of 2 and 9 to find out this)
Hence 1287 is not divisible by 18
Divisibility by 19
To find out if a number is divisible by 19, multiply the last digit by 2 and add it to the number formed by the remaining digits.
Repeat this process until you arrive at a smaller number whose divisibility you know.
If this smaller number is divisible by 19, the original number is also divisible by 19.
Example1: Check if 74689 is divisible by 19.
Given Number = 74689
7468 + (9 × 2 )= 7468 + 18 = 7486
748 + (6 × 2 ) = 748 + 12 = 760
76 + (0 × 2 ) = 76 + 0 = 76
76 is divisible by 19. Hence 74689 is also divisible by 19
Example2: Check if 71234 is divisible by 19.
Given Number = 71234
7123 + (4 × 2 )= 7123 + 8 = 7131
713 + (1 × 2 )= 713 + 2 = 715
71 + (5 × 2 )= 71 + 10 = 81
81 is not divisible by 19. Hence 71234 is not divisible by 19
Divisibility by 20
A number is divisible by 20 if it is divisible by 10 and the tens digit is even.
(There is one more rule to see if a number is divisible by 20 which is given below.
A number is divisible by 20 if the number is divisible by both 4 and 5)
Example1: Check if 720 is divisible by 20
720 is divisible by 10. (Please check the divisibility rule of 10 to find out this).
The tens digit = 2 = even digit.
Hence 720 is also divisible by 20
Example2: Check if 1340 is divisible by 20
1340 is divisible by 10. (Please check the divisibility rule of 10 to find out this).
The tens digit = 2 = even digit.
Hence 1340 is divisible by 20
Example3: Check if 1350 is divisible by 20
1350 is divisible by 10. (Please check the divisibility rule of 10 to find out this).
But the tens digit = 5 = not an even digit.
Hence 1350 is not divisible by 20
Example4: Check if 1325 is divisible by 20
1325 is not divisible by 10 (Please check the divisibility rule of 10 to find out this) though the tens digit = 2 = even digit.
Hence 1325 is not divisible by 20
Prime Numbers
A prime number is a positve integer that is divisible by itself and 1 only. Prime numbers will have exactly two integer factors.
Examples: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, etc.
Please note the following facts
Zero is not a prime number because zero is divisible by more than two factors. Zero can be divided by 1, 2, 3 etc.
(0 ÷ 1 = 0, 0÷ 2 = 0 …)
One is not a prime number because it does not have two factors. It is divisible by only 1
Composite Numbers
Composite numbers are numbers that have more than two factors. A composite number is divisible byat least one number other than 1 and itself.
Examples: 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, etc.
Please note that zero and 1 are neither prime numbers nor composite numbers.
Every whole number is either prime or composite, with two exceptions 0 and 1 which are neither prime nor composite
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