What is the probability of a couple having four sons?

If $x \log x \frac{dy}{dx} + y = \log x^2$ and $y(e) = 0$, then $y(e^2) = $

Gold numbers of protective colloids $A$,$B$,$C$ and $D$ are $0.50$,$0.01$,$0.10$ and $0.005$,respectively. The correct order of their protective powers is

$\frac{1}{2 \cdot 3} + \frac{1}{4 \cdot 5} + \frac{1}{6 \cdot 7} + \frac{1}{8 \cdot 9} + \dots$ is equal to

We can define a facultative parasite as one which:

What are seed-bearing plants called?