Answer:
90 days
Step-by-step explanation:
Let the rate per day for the 30 days period be x
Given that the rate drops to 1/3 of the rate used in 30 days period, it means that the new rate
= 1/3 x
If at a rate of x, the full tank runs for 30 days
at a rate of 1/3 x, the full tank would run for
= x/ (1/3 x) * 30 days
= 3 * 30 days
= 90 days
At the new rate, the full tank of oil will last for 90 days
