Answer:
Rapidly growing trade brought a lot of wealth and power to West Africa, just as the Ghana Empire was getting its start. The Ghana Empire, in particular, grew rich from the trans-Sahara trade. It certainly helped that the empire had control over the three major gold fields to the south of them.
THE ANSWER IS
GOING TO BE FALSE
assuming efficiency of each man to dig trench, in one day, for one hour is constant,

so one man, working for one hour in one day can dig 1.5 m
so 8 men, working 3 hours a day and will take, say $d$ number of days and efficiency will be constant. so,
$\frac32=\frac{220}{8\times3\times d}$
$\implies d=\frac{55}9=6.12$