A can do a work in 10 days and B in 20 days. If they together work on it for 5 days, then the fraction of the work that is left is
Let the total amount of work = 20 units
=> Rate at which A alone finishes work = $$\frac{20}{10}$$ = 2 units/day
=> Rate at which B alone finishes work = $$\frac{20}{20}$$ = 1 unit/day
=> Rate at which both A & B work together = 2+1 = 3 units/day
Work done by both in 5 days = 5*3 = 15 units
=> Amount of work left = 20-15 = 5 units
=> Fraction of work left = $$\frac{5}{20} = \frac{1}{4}$$
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