A moving coil galvanometer is converted into an ammeter, reading up to $0.04 \mathrm{~A}$ by connecting a shunt of resistance ' $3 \mathrm{r}$ ' across it and then into an ammeter reading up to $0.8 \mathrm{~A}$, when a shunt of resistance ' $r$ ' is connected across it. What is the maximum current which can be sent through this galvanometer if no shunt is used?