To find out how many hours it will take to stack all 3 cords of wood at the rate of 3/10 of a cord in 2/5 of an hour, you can set up a proportion:
(3/10 cord) / (2/5 hour) = (3 cords) / (x hours)
Now, cross-multiply and solve for x:
(3/10) * (5/2) = (3/x)
(3/10) * (5/2) = 15/20 = 3/4
So, it will take (3/4) of an hour to stack all 3 cords of wood at the given rate. You can also express this as 45 minutes since 3/4 of an hour is equivalent to 45 minutes.