Question

if ne person can do 25 sqare feet in 4 hours and another in 5 hours how much concrete can they do working together in 8 hours\
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Answer 1

rate of 1st person = 25/4 ft^2/h
rate of 2nd person = 25/5 ft^2/h
combined rate = (25/4+25/5) or 45/4 ft^2/h

so area in 8 hrs = 8(45/4) ft^2
= 90 ft^2

or

1st person can do 25 fT2 in 4 hrs, so he can do 50 ft^2 in 8 hrs
2nd person does 25/5 or 5 ft^2 in 1 hour , so in 8 hours he does 40 ft^2
50+40 = 90