Since price is constant, apply $TR=P\times Q$, $AR=TR/Q$, and $MR=\Delta TR/\Delta Q$ at each row.

$TR=25Q$ gives 25, 50, 75, 100, 125. $AR=TR/Q=25$ at every row. $MR$ = the constant difference between successive TR values = 25 at every row.

Since the firm sells at the fixed price of Rs. 25 regardless of quantity, $TR=P\times Q=25Q$:

$AR=TR/Q$ at each output level:

$MR=\Delta TR/\Delta Q$, the difference between successive TR values (taking $TR_0=0$):

The completed schedule is:

AR and MR are IDENTICAL to each other and to the price at every single output level — because the price never changes as output changes, every extra unit adds exactly the same Rs. 25 to total revenue, so both AR and MR stay locked at Rs. 25.

Students sometimes expect MR to be smaller than AR out of habit (from monopoly problems) — under perfect competition specifically, this is WRONG: because price is constant, AR and MR are always exactly equal, never merely close.