A certain number of men can complete a piece of work in $6k$ days, where $k$ is a natural number. By what percent should the number of men be increased so that the work can be completed in $5k$ days?

X, Y and Z can complete a piece of work individually in 6 hours, 8 hours and 8 hours respectively. However, only one person at a time can work in each hour and nobody can work for two consecutive hours. All are engaged to finish the work. What is the minimum amount of time that they will take to finish the work?

What percent of water must be mixed with honey so as to gain 20% by selling the mixture at the cost price of honey?

What is the rightmost digit preceding the zeros in the value of $30^{30}$?