$$D=800nm$$

$$V_1=?knots$$

$$V_2=0.8V_1$$

$$DC_1=Cons\:per\:day\:at\:V_1$$

$$DC_2=Cons\:per\:day\:at\:V_2$$

$$DC_1-DC_2=42t$$

$$C=\left(0.136+0.001V^3\right)\:t\h$$

$$Therefore\:DC=24\left(0.136+0.001V^3\right)\:tonnes\:per\:day$$$$42=24\left\lbrack\left(0.136+0.001V_{1^{}}^3\right)-\left(0.136_{}+0.001\left(0.8V_1^3\right)\right)\right\rbrack$$

$$42=24\left(0.136+0.001V_1^3-0.136-0.512\times10^{-3}\times V_1^3\right)$$

$$42=24\left(0.001V_1^3-0.512\times10^{-3}\times V_1^3\right)$$

$$42=24\left(0.000488V_1^3\right)$$

$$V_1=\sqrt[3]{\frac{42}{24\times0.000488}}$$

$$V_1=15.31\:knots$$

$$V_2=0.8\times V_1$$

$$V_2=0.8\times15.31$$

$$V_2=12.245\:knots$$

$$\left(i\right)\:Reduced\:cons\:per\:day\:=\:\left(0.136+0.001V_2^3\right)\times24$$

$$=\left(0.136+0.001\times12.45^3\right)\times24$$

$$=49.57\:tonnes\:per\:day$$

$$Time\:taken\:for\:complete\:voyage\:of\:800nm$$

$$at\:V_2=\frac{800}{12.245\times24}=2.72\:days$$

$$Consumption\:=\:2.72\times49.57=134.93t\:\left(at\:reduced\:speed\right)$$

$$\left(ii\right)\:Fuel\:onboard=134.93+50$$

$$=184.93t\:\left(after\:speed\:reduction\right)$$

$$DC_1=DC_2+42$$

$$DC_1=49.57+42$$

$$DC_1=91.57t$$

$$Time\:taken\:for\:V_1=\frac{800}{15.31\times24}$$

$$=2.178days$$

$$Cons\:at\:V_1=91.57\times2.178$$

$$=199.38t$$

$$\left(iii\right)\:\%\:reduction\:in\:cons=\frac{199.38-134.93}{199.38}$$

$$=32.32\%$$

$$\left(iv\right)\:\%\:increase\:in\:time=\frac{2.72-2.178}{2.178}$$

$$=24.88\%$$