$\text{Total work} =\text{Efficiency (or) One day work}\times \text{Total time}$
$\textbf{W = DMTE}$
D = Number of days
M = Number of men
T = Number of hours per day
E = Efficiency
W = Amount of work
Now, $D_{1}M_{1} = D_{2}M_{2}$
$\implies 30\times A = 40\times B$
$\implies 3A =4B\implies B = \dfrac{3A}{4}$
Again, $D_{1}M_{1} = D_{2}M_{2}$
$\implies 30 \times A = x \times (A+B)$
$\implies 30A = x \times \bigg(A+\dfrac{3A}{4}\bigg)$
$\implies 30A = x \times \dfrac{7A}{4}$
$\therefore x = \dfrac{120}{7}$ days