@ankit
Transaction 1 (T1) does not anything about the Transaction 2 (T2).
Lets say T1 and T2 both wants to add 10 and 20 to the variable, M whose current value is 100.
T1 comes first, reads M (=100), adds 10, makes its 110 in its own local copy. Before commit, it goes to sleep.
T2 comes, reads M (=100), adds 20, makes its 120 an commits this. M is now 120.
T1 wakes up, and finishes its commit by saving its local M value, which is 110. After commit M becomes 110.
So essentially the commit done by the T2, when its makes the M 120 is lost.