In a school, there are $20$ teachers who teach Mathematics or Physics. Of these, $12$ teach Mathematics and $4$ teach both Physics and Mathematics. How many teach Physics?

Let $\underset{i=1}{\overset{50}{\cup }}{X}_i=\underset{i=1}{\overset{n}{\cup }}{Y}_i=T$, where each $X_i$ contains $10$ elements and each $Y_i$ contains $5$ elements. If each element of the set $T$ is an element of exactly $20$ of sets $X_i$'s and exactly $6$ of sets $Y_i$'s then $n$ is equal to :

In a school, there are three types of games to be played. Some of the students play two types of games, but none play all the three games. Which Venn diagrams can justify the above statement?

Let $S_1, S_2$ and $S_3$ be three sets defined as

$S_1=\{z\in ℂ:|z-1|\le \sqrt{2}\},$

$S_2=\{z\in ℂ:Re((1-i)z)\ge 1\}$ and

$S_3=\{z\in ℂ:Im(z)\le 1\}.$

Then, the set $S_1\cap S_2\cap S_3$