To find out what percentage of the total cost Vinh has saved, you can use the formula:
\[ \text{Percentage Saved} = \left( \frac{\text{Amount Saved}}{\text{Total Cost}} \right) \times 100 \]
Substituting in Vinh's values:
\[ \text{Percentage Saved} = \left( \frac{160}{400} \right) \times 100 \]
Calculating this:
\[ \text{Percentage Saved} = 0.4 \times 100 = 40% \]
Vinh has saved 40% of the cost of the standing mixer.