Present st calculation method

A individual security service provider previously raised bill to a company for providing cleaning staff is hereunder-

Bill-                  Rs. 1,00,000

ST @ 12.36-    Rs.   12,360

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Total-             Rs. 112,360

Now he is providing as follows-

Bill-                  Rs. 1,00,000

ST @ 12.36-    Rs.       3090 (i.e. 25% of Rs. 12,360)

____________________

Total-             Rs. 103,090

Please confirm me whether the the present calculation right as per security service provider and what will be the basis of calculation of this 25% of ST.

Advance Thanks to you all.