How did the Spanish-American War help the United States become a world power?
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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,
$\frac32=\frac{220}{8\times3\times d}$
$\implies d=\frac{55}9=6.12$