Inserting The Missing Character Questions

In the following question, by using which mathematical operators will the expression become correct?
15 _ 3 _ 4 _ 20

Select the missing numbers from the given alternatives

In the following question, which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?

W_YZ_X_ZW_Y_

In the following question, from the given alternative words, select the word which cannot be formed using the letters of the given word.
Nightwalker

Select the missing number?

Which set of letters when sequentially placed at the gaps in the given letter series shall complete it?

j_l_jk_m_kl_

A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.

A, D, I, P, ?

Which set of letters when sequentially placed at the gaps in the given letter series shall complete it?

A_BC_BBC_BB_