Hello Deepika

The initial amount of substance = N(0)

The amount of substance after time (t) = N

Given - 

Half time of the substance (T) = 30 minutes

Total time passed (t) = 2 hours

Convert the hours into minutes.

2 hours = 2 × 60 minutes 

              = 120 minutes.

The fraction for total number of atoms decayed is

                    N/N(0) = (1/2)^(t/T)

                                 = (1/2)^(120/30)

                                 = (1/2)^(4)

                                 = (1/16)

                             N = N(0)/16   

Thus the fraction of decayed atoms will be 1/16. And the amount remaining will be 1/16 of original amount.