Answer:
17.5 days
Explanation:
1/5 box cereal per day
3 1/2 box cereal total
convert to improper fraction: 3 1/2 = ((3*2)+1)/2= 7/2
now you have:
1/5 box cereal per day
7/2 box cereal total
convert to common denominator (same bottom number in fraction, which makes it easy to evaluate later):
1/5 = 2/10 box cereal per day
(7*5)/(2*5)= 35/10 box cereal total
total amount/ amount used per day = how many days the thing will last
(35/10)/(2/10) = 35/2 = 17.5
when same denominator (bottom number in fraction) in fractions that are divided by each other you can just cross out the bottom number
will last 17.5 days