X is twice as good as Y and Y is twice as good as Z while doing a task. If X and Y can complete the task in 4 days, then how many days will z take to complete that task himself only.

24 days

                 X   :   Y   :   Z

Efficiency:  4  :   2   :    1

Now,

                 X + Y   :    Z

Efficiency:     6     :    1

Time       :     1     :    6

ATQ,

(X + Y )1R = 4 days

Therefore, Z alone will do in = 6R = 6 × 4 = 24 days