first off, the "elapsed time" unit and the "rate" unit must correspond, in this case the decay rate is 4% per second, whilst the elapsed time is in minutes, so let's change that to seconds, hmmmm there are 60 seconds in 1 minute so in 0.8 minute there are 0.8*60 = 48 seconds.
[tex]\qquad \textit{Amount for Exponential Decay} \\\\ A=P(1 - r)^t\qquad \begin{cases} A=\textit{current amount}\\ P=\textit{initial amount}\dotfill &920\\ r=rate\to 4\%\to \frac{4}{100}\dotfill &0.04\\ t=\textit{elapsed time}\dotfill &48\\ \end{cases} \\\\\\ A=920(1-0.04)^{48}\implies A=920(0.96)^{48}\implies A\approx 129.66[/tex]
