Express the number of gallons of water in the barrel, A, as a function of the elapsed time, t, in hours...?

The amount of water in a barrel is always decreasing by a constant percent of its current value (the water leaks out at a continuous rate). Initially, there were 250 gallons of water in the barrel. After 4 hours there were 213 gallons in the barrel. Express the number of gallons of water in the barrel, A, as a function of the elapsed time, t, in hours. Rond to the nearest hundredth.