In the expression $5 * 4 * 3 * 2 * 1$, * is chosen from $+$, $-$, $\times$ each at most two times. What is the smallest non-negative value of the expression?

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?

How many consecutive zeros are there at the end of the integer obtained in the product $1^2 \times 2^4 \times 3^6 \times 4^8 \times \ldots \times 25^{50}$?