The common difference of the AP is

A contract on construction job specifies a penalty for delay of completion beyond a certain date as follows: Rs. 200 for the first day, Rs. 250 for the second day, Rs. 300 for the third day, etc., the penalty for each succeeding day being Rs. 50 more than for the preceding day. How much money the contractor has to pay as penalty, if he has delayed the work by 30 days?

The common difference of the AP is

The next term of the AP is

If 4, x₁, x₂, x₃, 28 are in AP then x₃ = ?