If ( 0.4x + 1/x ) = 5, what is the value of ( 0.064x^3 + 1x^3 )?

Answer: Option C

$$\begin{array}{rl}& \left(0.4x+\frac{1}{x}\right)=5\\ & \text{cube both side}\\ & {\left(0.4x\right)}^3+\frac{1}{x^3}+3\left(0.4x+\frac{1}{x}\right)\times \left(0.4x\right)\left(\frac{1}{x}\right)={\left(5\right)}^3\\ & 0.064x^3+\frac{1}{x^3}+3\left(5\right)\left(0.4\right)=125\\ & 0.064x^3+\frac{1}{x^3}+6=125\\ & 0.064x^3+\frac{1}{x^3}=119\end{array}$$