Something like this should work:
SELECT
A.Sno,
A.ColA,
A.ColB,
A.ColC,
Round(100. * A.ColC / B.ColC, 2) As Perc
FROM
YourTable As A
INNER JOIN YourTable As T
ON T.ColA = A.ColA
And Len(T.ColB) = 2
And T.ColB = STUFF(T.ColA, 2, 1, 'T')
;
Output:
Sno ColA ColB ColC Perc
-----------------------------------------------
1 AA A01 10 12.50000000000
2 AA A02 25 31.25000000000
3 AA A03 15 18.75000000000
4 AA A04 30 37.50000000000
5 AA AT 80 100.00000000000
6 BB B01 20 30.77000000000
7 BB B02 20 30.77000000000
8 BB B03 15 23.08000000000
9 BB B04 10 15.38000000000
10 BB BT 65 100.00000000000