Question

Farm Joe ordered 3 bags of soil last month.each bag weighed 4 2/5 kilograms. He used the first bag in a week. At the end of this month, there were 2 3/4 kilgrams of soil left in the second bag and 7/8 kilogram of soil left in the third bag. how much soil was used in this month?

Answer 1

9 23/40 kg

Explanation:

The three bags ordered weighed a total of

... 3 × 4 2/5 = 12 6/5 = 13 1/5 . . . kg

The amount remaining at the end of the month was

... 2 3/4 + 7/8 = 2 6/8 + 7/8 = 2 13/8 = 3 5/8 . . . kg

The difference is the amount used.

... 13 1/5 - 3 5/8 = (13 -3) + (1/5 -5/8) = 10 + (1*8 -5*5)/(5*8)

... = 10 - 17/40

... = 9 23/40 . . . kg

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The sum or difference of fractions can always be computed from ...

... a/b ± c/d = (ad ± bc)/(bd)

In this case, bd = 5·8 = 40 is the least common denominator.

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A calculator will often handle problems like this very nicely.