If you spend 9 kg of coal daily for heating, then it will be enough for 72 days

If you spend 9 kg of coal daily for heating, then it will be enough for 72 days, for how many days will coal last, if you spend 8 kg daily?

1. Let’s find the mass of coal that was consumed in 72 days:

9 kg x 72 = 648 kg.

2. Let’s find for how many days 648 kg of coal will last if 8 kg are consumed daily.

648 kg: 8 = 81.

Answer: if you spend 8 kg of coal every day, then 648 kg will be enough for 81 days.