7 men can complete a piece of work in 12 days. How many additional men will be required to complete double the work in 8 days ?

In 12 days unit work is done by 7 men
hence in 1 day $$\frac{1}{12}$$ amount of work is done by 7 men
so 1 man does $$\frac{1}{12\times7}$$ amount of work in one day.
we have to find no of man to do double the work in 8 days with above efficiency
let's say we have $$x$$ men
now total work done in single day will be $$\frac{1}{12\times7} \times x $$
hence in 8 days = $$\frac{1}{12\times7} \times 8x $$ = 2 ( as work is doubled)
$$x$$ = 21
additional man required = 14