Find all pairs of consecutive odd positive integers,both of which are smaller than $18$,such that their sum is more than $20$.

In the first four examinations,each of $100$ marks,Ravi scored $94, 73, 72,$ and $84$ marks. If a final average greater than or equal to $80$ and less than $90$ is needed to obtain a final grade $'B'$ in a course,find the minimum marks that Ravi must obtain in the fifth examination to get grade $'B'$.

$A$ solution of $8 \%$ boric acid is to be diluted by adding a $2 \%$ boric acid solution to it. The resulting mixture is to be more than $4 \%$ but less than $6 \%$ boric acid. If we have $640$ litres of the $8 \%$ solution,how many litres of the $2 \%$ solution will have to be added?

The length of a rectangle is five times the breadth. If the minimum perimeter of the rectangle is $180 \ cm$,then:

The set of all real values of $x$ satisfying the inequalities $x^2-4x+3 > 0$ and $x^2-2x-8 \leq 0$ is

The solution set contained in $R$ of the inequation $3^x+3^{1-x}-4 < 0$ is: