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. 

Let's look at multiples of $3$ between $2$ and $13$. There are $3, 6, 9, 12$, which is four of them. But $4 \times 3 = 12$, and between $2$ and $13$ is only $11$ numbers. 
The problem is that the number $4 \times 3$ doesn't represent the length of the interval containing the four multiples. There are only three gaps of length $3$, plus one more for the last number. That's a length of ten, which fits easily inside the $11$ spaces you've got. 
