Let the required number of days be x.

If amount of rice is more it will last for more days.
∴ It is Direct Proportion.
∴ Multiplying factor is 60/420
If men increases number of days the rice lasts decreases
∴ It is an inverse proportion.
∴ Multiplying factor is 98/42
x = 45 × 60/420 × 98/42

x = 15 days