Question

a garrison of 620 men had provision for 42 days.a reinforcement of 220 men arried. how long the food will last

Answer 1

Total man = 620+220 = 840
Now , By using Unitary method,
If 620 man had provision for 42 days.
So, For 840 man ,it will last for
42*620/840 = 31 days.

Answer 2

Men in the garrison = 620
they had provision for 42 days.
So units of provision present = 620×42

extra men = 220

now, total men in garrison = 620+220 = 840

Provision will last =
$\frac{620 \times 42}{840} = 31 \: days$

Provision will last only for 31 days.