Step 1: Define Efficiencies
Let Chandan's efficiency be $x$ units/day.
Bipin is twice as efficient as Chandan $\implies$ Bipin's efficiency = $2x$.
Ankita is twice as efficient as Bipin $\implies$ Ankita's efficiency = $2(2x) = 4x$.
Efficiency Ratio $A:B:C = 4:2:1$.

Step 2: Calculate Total Work
• For the first 20 days, all three worked: $(4x + 2x + x) \times 20 = 7x \times 20 = 140x$.
• After 20 days, Bipin leaves. Total time is 60 days, so remaining time = $60 - 20 = 40$ days.
• For the remaining 40 days, Ankita and Chandan worked: $(4x + x) \times 40 = 5x \times 40 = 200x$.
Total Work = $140x + 200x = 340x$.

Step 3: Chandan's Individual Time
Time taken by Chandan alone = $\frac{\text{Total Work}}{\text{Chandan's Efficiency}}$
$\text{Time} = \frac{340x}{x} = \mathbf{340}$ days.