book Maths Puzzle Numerical Ability

If John paints a house in 4 hours and Cindy paints a house in 6 hours, how long would it take them if they painted the house together??
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book Other Question

If a,b,c are in Arithmetic progression and a^2,b^2,c^2 are in Harmonic progression then what is the relation connecting a,b,c? Can you pl;ease tell me if I have done this problem correctly? I just feel like is is off. The problem is as follows:

A sum of $2000 is split between two investments, one paying