No. of farmers = A, no. of days = B & area of land = C

No. of days is in inverse variation with no. of farmers, when area of land remains constant

i.e., B ∝ \frac{1}{A} when C is constant

Again, No. of days is indirect variation with area of land; when No. of farmers remains constant

∴ B ∝ C when A is Constant.

According to the theorem on joint variation,

B ∝ \frac{C}{A} when C & A both vary

∴ B = K \frac{C}{A} where K is a constant of variation.

Given A = 15, B = 5, & C = 18.

5 = K \frac{18}{15}

or, K =   \frac{15 \times 5}{18} = \frac{25}{6}

⇒ B = K \frac{C}{A}

⇒ B = \frac{25}{6} \times \frac{12}{10} = 5

∴ No. of days = 5.