QR need help~

[joonkimdds]

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propane last 60 days during a normal winter.
Propane use increased by 25% this winter.
how many days will the same amount of propane last?

I set it up as following
I assumed that the amount of propane is 100 and subtracted 25 because
propane use increased by 25% so I think we can assume that we have 25% less.

100 = 60 days
75 = x days

x = 60*75 /100 = 45days

however, the correct answer is 48 days.

Could someone tell me why my method doesn't work?

 

[UCB05]

propane last 60 days during a normal winter.
Propane use increased by 25% this winter.
how many days will the same amount of propane last?

I set it up as following
I assumed that the amount of propane is 100 and subtracted 25 because
propane use increased by 25% so I think we can assume that we have 25% less.

100 = 60 days
75 = x days

x = 60*75 /100 = 45days

however, the correct answer is 48 days.

Could someone tell me why my method doesn't work?

60/1.25= 48
Your error was in your setup. Using 25% more per day does not equal having 25% less. What if the usage had increased by 100%? By your method, he would have no propane to start.

Correct setup: X= amount (gallons, pounds,etc.) of propane, R=original rate of usage in amount/day, 1.25R= increased rate of usage, D= days at 1.25R rate of usage:
X= R (60)= 1.25R (D)>>> R (60)= 1.25R (D)>>> D=60/1.25= 48 days
X= (R)