divisibility by 21 It's a simple problem but I am stuck.
The multiple of 21 between 700 and 950 are 714, 735, 756, 777, 798, 819, 840,861,882,903,924,945.
So, there are 12 multiples of 21 between 700 and 950 but 21 *12=252.
So 12 multiples of 21 should take an interval of 252 , and there are only  250 numbers between 700 and 950, So there should be only 11 multiples .
Where am i making a wrong argument ?
Please help.
Thanks. 

You are wrong when you claim that “$21\times12=252$, so $12$ multiples of $21$ should take an interval of $252$”. The numbers $12$ and and $24$ are two consecutive multiples of $12$, but they take an interval of $13$ numbers only. 
