The height of A is 20% more than of B, and the height of B is 20% less than that of C. The height of C is 30% more than that of D. If the difference between the heights of C and A is 6.8 cm, then what is the height (in cm) of B?
X can do a certain work in 40 hours. Y is 25% more efficient than X, and Z is 28% more efficient than Y. Working together, X, Y and Z will complete the same work in: