Please Help me sove for C

Suad Alwan the purchasing agent for Dubai Airlines, has determined that the third place took $18000$ hours to produce. using an $80\%$ learning curve and a $$30-$per$-$hour labor charge, he wants to determine the cost of the four additional planes.

A$.)$ Time required for the fourth unit $=16,408$ hours $($round your response to the nearest whole number$).$

B$.)$ Cost of the fourth unit $=492240$ dollars $($round your response to the nearest whole number$).$

C$.)$ Time required for the fifth unit $=$ hours $($round your response to the nearest whole number$).$