After a hot shower and dishwashing, there is not hot water left in the 50-gal (185-L) water

Question:

After a hot shower and dishwashing, there is “not hot water” left in the 50-gal (185-L) water heater. This suggests that the tank has emptied and refilled with water at roughly 10oC.
(a) How much energy does it take to reheat the water to 50oC?
(b) How long would it take if the heater output is 9500 W?