Answer:
96
Explanation:
There are 2021 numbers and subtracting 7 each time, we will be left with 2021/7 ≈ 289 numbers
2021/3 = remainder 2
⇒2021 = 3x + 2
2021 - 7 = 3x + 2 - 7
= 3x - 5
= 3x - 3 - 2
= 3(x - 1) - 2
adding and sub 3,
= 3(x - 1) - 2 + 3 - 3
= 3(x - 1 - 1) + 1
=3(x - 2) + 1
The remainder is 1 and hence not divisible by 3
2021 - 7 - 7 = 3(x - 1) - 2 - 7
= 3(x - 1) - 9
= 3(x - 1 - 3)
= 3 (x - 4)
Remainderis 0 and hence divisible by 3
So, every 3rd number in the 289 numbers is divisible by 3
⇒ 289/3 ≈ 96
Cog counted 96 numbers