Solve this $90:30::36:x$.

Hint: This is a kind of word problem which confuses students with different signs in the questions. Here we go with the simple concept of mathematics which is called proportion, the problem is so simple but signs are so confusing.

Complete step by step answer:
So, whenever we see four numbers a, b, c and d in proportion like $a:b::c:d$, then convert this unusual looking proportion into $\dfrac{a}{b}=\dfrac{c}{d}$ which have same meaning like $a:b::c:d$, but some people rewrite this proportion like $a\times d=b\times c$ in which they multiplied a and d which is called as product of extreme terms $\left( i.e.\ a\times d \right)$ and equate to multiplication of b and c which is called as product of middle terms $\left( i.e.\ b\times c \right)$.
Therefore, $a:b::c:d$
$\Rightarrow a\times d=b\times c$
So, here $90:30::36:x$ can be rewrite as,
$\begin{align}
  & 90:30=36:x \\
 & \dfrac{90}{30}=\dfrac{36}{x} \\
 & x=\dfrac{36\times 30}{90} \\
 & x=12 \\
\end{align}$
So, $x=12$will be the correct answer.

Note: Sometimes the problem has no sign, it just gives you four numbers and says the numbers are in proportion and asks you to find out the $x$ which is variable in given four numbers.