PrakharGMAT wrote:
Hi
AdlaT,
Sorry to say but I didn't got your approach.
Actually, I have seen an approach in which big numbers are given eg- 737 to 943 (Just a random range)
And then we break these no into something...In the end we get an range which is below 100 and we just need to calculate the prime no between them..
Nut unfortunately I am not able to recall that approach.
Your approach seems to be quiet similar
As you just calculate the prime no.s between 20 and 40
How did you reached to this thing.
Total No of prime numbers between 20 to 40 = Total No of prime numbers between 260 to 280
Can you please assist..?
I will try to explain my strategy.
260 and 280 can be expressed as product of smallest possible integers plus smallest possible integer.
because i know exactly what are the prime numbers less than 100, which makes problem easy for me.
260 can be expressed in smallest possible integers as 2*2*3*4*5+20
and 280 can be expressed as in smallest possible integers as 2*2*3*4*5+40.
now you can see the base value which is 2*2*3*4*5 is common in both values, now you can find the prime numbers easily from 20 to 40.
Your Question.
How many prime numbers exist between 737 and 943?
Solution:
737 Can be expressed as 6!+17.
943 can be expressed as 6!+ 223. base should be the same in minimum and maximum numbers.
in this case i need to know prime numbers greater than 100, which is time consuming. i will break this range in to range of 100.
so 6!+17(=737) to 6!+100(820)--> prime numbers between 17 and 100 are 19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97=18.
But you should also consider 6!+49, 6!+77 and 6!+91-->Total 18+3=21 prime numbers between 737 and 820.
now Between 820 and 840.7*5!-0+to 7*5!-20 , you can not factor out 11,13,17, and 19--4 prime numers,
Now between 840 and 943
7*5!+0 to 7*5!+101 ---total prime numbers between 0 and 103 are 26.
Total prime numbers between between 737 and 943=21+4+26=51.
hope it helps.