60 should be the correct answer.
# Since cross sectional area of any cylinder is never going to change, the time required to make a cut will always be constant for both manual and mechanical saws.
# To cut 1 log into 4 equal cylindrical pieces, we have to make 3 cuts, so to cut 60 logs each into 4 equal cylindrical pieces, we have to make 60 x 3 = 180 cuts total.
#we have sufficient man power(2 + 2 + 4 + 4) to run all the saws in parallel.
# manual saws requires time twice then time required by mechanical saws, so 1/3rd of the cuts(=60) will be made by manual saws and 2/3(= 120) will be made by mechanical saws. Thus each manual saw has to make 30 cuts and each mechanical saw has to make 60 cuts, all running in parallel.
#the time required to make 30 cuts by manual saw(2 x 30) = the time required to make 60 cuts by mechanical saw(2 x 60) = 60.
So this overall process can be done, most efficiently in 60 hours.