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Option 2 : 20%
Calculation:
Let number of days taken by P alone and by Q alone to complete the work be p days and q days respectively.
P's 1 day's work = 1/p
Q's 1 day's work = 1/q
Work completed in first four hours = 1 - 3/5 = 2/5
⇒ 4/p + 1/q = 2/5 ----(1)
P completes 2/3 of work and Q completes (1 - 2/3 = 1/3) of work.
⇒ 2p/3 - q/3 = 3 ----(2)
Solving (1) and (2),
⇒ p = 12 and q = 15
P and Q alone complete the work in 12 days and 15 days respectively.
∴ Required percentage = (15 - 12)/15 × 100 = 20%
For the first four hours, P works for all four hours while Q works for only one hour.
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