Sanket earns twice as much in the month of March as in each of the other months of the year. What part of his entire annual earnings was earned in March?

Let Sanket's earnings in each of the other $11$ months of the year be $x$.
According to the problem, his earnings in the month of March is twice that amount, which is $2x$.

Let's compute the total annual earnings:

$$\text{Total Annual Earnings}=(\text{Earnings in March})+(\text{Earnings in remaining }11\text{ months})$$

$$\text{Total Annual Earnings}=2x+11\times x=2x+11x=13x$$

We need to find the fraction of his annual earnings that was earned in March:

$$\text{Required Fraction}=\frac{\text{Earnings in March}}{\text{Total Annual Earnings}}=\frac{2x}{13x}=\frac{2}{13}$$