The sum of all prime numbers less than 20 is:

A prime number is a number that can only be divided by 1 and itself.

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Checking each number from 2 to 19:

2 is prime (only divisible by 1, 2)

3 is prime (only divisible by 1, 3)

4 is not prime (divisible by 1, 2, 4)

5 is prime (only divisible by 1, 5)

6 is not prime (divisible by 1, 2, 3, 6)

7 is prime (only divisible by 1, 7)

8 is not prime (divisible by 1, 2, 4, 8)

9 is not prime (divisible by 1, 3, 9)

10 is not prime (divisible by 1, 2, 5, 10)

11 is prime (only divisible by 1, 11)

12 is not prime (divisible by 1, 2, 3, 4, 6, 12)

13 is prime (only divisible by 1, 13)

14 is not prime (divisible by 1, 2, 7, 14)

15 is not prime (divisible by 1, 3, 5, 15)

16 is not prime (divisible by 1, 2, 4, 8, 16)

17 is prime (only divisible by 1, 17)

18 is not prime (divisible by 1, 2, 3, 6, 9, 18)

19 is prime (only divisible by 1, 19)

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Prime numbers less than 20: $2, 3, 5, 7, 11, 13, 17, 19$

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$2 + 3 + 5 + 7 + 11 + 13 + 17 + 19$

$= (2 + 3) + (5 + 7) + (11 + 19) + (13 + 17)$

$= 5 + 12 + 30 + 30$

$= 77$

Therefore, the sum of all prime numbers less than 20 is $77$.