TIME AND WORK

1) 5 People take 8 days to finish a work. How many days need finish the same work by 4 people?
in Trick :
People         Days
5        -        8
4        -        x
X = 5 X 8 / 4  =  10 Days

2) 10 people take 15 days to finish a work. How any days need to finish the same work by 6 people?
People         Days
10      -        15
6        -        x
X = 10 X 15 / 6  =  25 Days