we have that
four people, five hours a day, six days
that represents
each people works-----> 30 hours in six days (5*6)
each people can do ----> 25% of the work (100%/4)
Divide percentage by the number of hours
25/30=5/6 % by hour
that means
Each people can do (5/6)% by hour
two people working for seven hours a day--------> represents 14 hours a day
Multiply 14 hours by (5/6)
14*(5/6)=11.67%
that means
two people in one day can do 11.67% of the work
Applying proportion
1/11.67%=x/100%
solve for x
x=100/11.67
x=8.6 days------> Round up
the answer is 9 days