Hi,
I also had similar doubt before. Let me explain how it works.
Series with 1 as remainder after dividing by 7 = 8, 15, 22, 29, 36,.....
Now, if we see among the above series which numbers give 2 as a remainder, we can get the below series.
Dividing the above series by 3 and see if the remainder is 2 =
R(8/3)=2, R(15/3)=0, R(22/3) =1,
R(29/3) = 2.....
Note in R(8/3)=2, 2 is the remainder when 8 is Divided by 3
So the remainder cycle is =
2, 0, 1,
2, 0, 1.....
Note: The first number of the cycle gives the remainder 2 (it is important, I will explain why)Since we know there are 71 numbers below 500 that are divisible by 7. 499/7 = 71.
We want a number that will give 1 as the remainder after dividing by 7.
So the first number in our series will be 8, and the last will be 498.
And we already know the remainder cycle is 3. So we need to divide the number 71/3= 23.66
Now, as we know, up to 69th number among 71, there will be 69/3 =23 numbers. But in the remaining 70th and 71st numbers, the 70th number will also give remainder 2 since our cycle starts with remainder 2 (as I mentioned before)So the extra number is coming from 70th number from the series. So total number = 23 +1 = 24I hope it helps.
valeriugirlea
Why do we ad 1 to 23 in the end?