If it takes 6 men 4 days to dig a hole 3 feet deep, how long will it take 10 men to dig a hole 7 feet deep?

Now, i am up to the part that it would take one man 8 days to dig one foot

stuck from there on how to make an equation

i know the answer would be 5.6 (or 6 to the nearest day)

as (7 x 8) /10 = 5.6

but i am looking for a neat formula to use for future problems