Sequences and S...

If AM of two numbers a and 17 is 15. Then a is ___

Arithmetic mean of two numbers a+d and a-d is 

Find the sum of Arithmetic means of $$3 , 9$$ and $$12 , 8$$.

The arithmetic mean of $$4$$ and $$14$$ is 

Insert an arithmetic mean between $$7$$ and $$21$$

The number of distinct rational numbers x such that $$\displaystyle 0 < x < 1$$ and $$\displaystyle x = \frac{p}{q}r$$, where $$\displaystyle p,q \epsilon \left \{ 1,2,3,4,5,6 \right \}$$,is

The number of rectangles that can be obtained by joining four of the twelve vertices of a $$12$$ sided regular polygon is

Three vertices are chosen randomly from the seven vertices of a regular $$7$$ -sided polygon. The probability that they form the vertices of an isosceles triangle is

The number of ways in which three numbers in A.P. can be seleced from the set of first n natural number if n is odd is