1 Jack is painting twice as fast as Tom.
2 If Tom painted twice as fast as he actually does, the brothers would finish the work in 3 hours
Explanation:
Statement (1) by itself is sufficient. S1 reduces to equation 1/J + 1/(2J) = 1/4, where J denotes the time it takes Jack to paint the wall alone. From this equation J=6
Statement (2) by itself is sufficient. S2 reduces to equation: 1/J + 2*(1/4 – 1/J) = 1/3 ; J=6
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When you say Jack painted twice as fast as Tom it means - Toms time is double or rate is 1/2 - explanation for 1 is fine.
When you move on to 2: If Tom painted twice as fast as he actually did - maintaining the same symbology as in statement 1, statement 2 should be
1/J + 1/2(1/4-1/J) = 1/3. (With this equation you dont get a unique solution using statements 1 and 2 individually).
I understand 1/J and (1/4-1/J )are rates and twice as fast means work is done in half the time or at double the rate. It would have been nice if the problem statement/solution were consistent !