Answer:
9 23/40 kg
Step-by-step 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.
