If x men can do a piece of work in x days, then the number of days in which y men can do the same work is -

\(\frac{y}{x}\)      xy  \(\frac{x}{y}\)       \(\frac{x²}{y}\)

\(\frac{x²}{y}\)

Men × Days   = constant (when work is same)  xM  ×  x    =   yM  × no. of days  ⇒ No. of days = \(\frac{x^2}{y}\)