What are the limitations & advantages of BCD & binary number systems?

The value of $101010$ into the decimal number is ?

Represent the union of two sets by Venn diagram for each of the following.

$X=\{x | x$ is a prime number between $80$ and $100\}$

$Y=\{y | y$ is an odd number between $90$ and $100\}$

In a five digit number, the digit at ten's place is $8$, digit at units place in one-fourth of the digit at ten's place, the digit at thousands place is $0$ digit at hundreds place is twice that of units place and the digit at ten thousands place is thrice the digit at unit's place. What is the number?

The sum of the digits of a certain two digit numbers is $9$. If $9$ is added to the number, the digits are reversed. The number is

$$\begin{gathered} \left(A\right) 36 \\ \left(B\right) 27 \\ \left(C\right) 45 \\ \left(D\right) 18 \end{gathered}$$