Question

write three pairs of prime numbers less than 20 whose sum is divisible by 5

Answer 1

The pairs of numbers are (7, 13) and (13, 17) and (19, 11)

Solution:

Given that we have to write three pairs of prime numbers less than 20 whose sum is divisible by 5

Let us first list the prime numbers less than 20

A prime number is a whole number greater than 1 whose only factors are 1 and itself

Prime numbers less than 20 are:

2, 3, 5, 7, 11, 13, 17, 19

We have to find pairs of prime numbers in above list whose sum is divisible by 5

Number that are divisible by 5 must end in 5 or 0

7 + 13 = 20 = divisible by 5

13 + 17 = 20 = divisible by 5

19 + 11 = 30 = divisible by 5

Thus the pairs of numbers are (7, 13) and (13, 17) and (19, 11)

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Answer 2

Answer:

$\red { Required \: pairs \: are ,}$

$\green { (3,17),(3,7),(7,13),(13,17) }$

Step-by-step explanation:

$\underline { \pink { Prime \: number }}$

A natural number greater than 1 is said to be a prime , if it has only two factors i.e 1 and itself .

2,3,5,7,11,13,17,19 are prime numbers less than 20.

$\underline {\green { Pairs \:of \:prime \: numbers }}$

$\underline {\green { less \:than \:20 \: whose \:sum \: is }}$

$\underline {\green { Divisible\:by \:5\:are :}}$

$i ) 3 + 17 = 20 \\ii) 3 + 7 = 10 \\iii ) 7 + 13 = 20 \\iv) 13 + 17 = 30$

Therefore.,

$\red { Required \: pairs \: are ,}$

$\green { (3,17),(3,7),(7,13),(13,17) }$

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