In a fort, 360 men had provisions for 21 days. If 60 more men join them, how long will the provisions last?

Original men = 360.

New men = 360 + 60 = 420.

Provisions for 360 men last = 21 days.

Provisions for 1 man last = (21 x 360) days. $\quad$ [Less men, More days]

Provisions for 420 men last = $\Big(\dfrac{21 \times 360}{420}\Big) \text{ days} \quad \text{[More men, Less days]}$

= $\Big(\dfrac{1 \times 360}{20}\Big) \text{ days} \quad \text{[Dividing 21 and 420 by 21]}$

= 18 days.

420 men had provisions for 18 days.