Question

The sum of the prime numbers less than x is 58. find an integer value of x.

Answer 1

that no.will be 18
since 2+3+5+7+11+13+17=58

Answer 2

if you are solving past papers from 0580/23/m/j/13, then you probably know already that the number has to be less than 21, for the sum of prime numbers below 21 is 77. you have to start by subtracting the bigger numbers in the set. for example, 77-19= 58.

here you got the answer immediately because of the set of numbers you already had, (2+3+5+7+9+11+13+17+19)

and now the highest number you have is 17, so x= 18.

of coarse you can always just add the numbers one by one, but its easier to remember the following:

sum of numbers <10 = 17

sum of numbers 10-20 = 60

sum of prime numbers 20-30 = 52

this way helps a lot when you don't have much time to calculate them separately during a test for example.

i hope this was helpful!