Answer
$ R= 4.9 \times 10^{13} Bq$
Work Step by Step
Solving the number of nucleus from mass of sample,
$ N = \frac{M_{Sample} }{M_{Kr}} N_A$
$ N = [\frac{(20 \times 10^{-9} g)}{92 g/mol] }] (6.022 \times 10^{23}/mol)$
$ N = 1.30 \times 10^{14} $
From here we can find the activity (rate) of the decay
$ R = \frac{N ln2}{T_{1/2}}$
$ R= \frac{(1.30 \times 10^{14})(0.693) }{1.84 s} $
$ R= 4.9 \times 10^{13} Bq$
