The pattern is as follows :
B+3 ? E+4 ? I+5 ? N+6 ? T+7 ? A

The sequence is follow backward direction of the English alphabet order
Z-3 ?W-4?S-3?P-4?L-3?I-4?E-3?B

In ABA, middle letter is +1 of first and third letter i.e., A in alphabetical order. Again in ECE, the middle letter is -2 of the first and the third letter i.e., E in alphabetical order. So, in CDC middle letter is +1 and GEG the middle letter is -2.

In each pair, three letters are between them like CG in between C and G three letters are there i.e., D, E and F. So, GK will fill the blank.

Except insects all other fly in the sky.

Three meaningful words are formed from letter EML and the words are MEAL, LAME and MALE.

After forming the new ser of letters according to the question only one meaningful word can be formed i.e., ALONE. So, the midle letter is O.

A person sits on Chair and here Chair is called as Cot. So, the person sits on Cot.

100 = 5 x 20, 64 = 4 x 16, 80 = 4 x 20 and required number will be 3 x 16 is 48

relationship is, (a² -1 ) : [(a + 4)² + 1]