400lbs/340ft^2
Factor out largest common factor. LCF is the product of shared primes of the numbers prime factorization.
400=2*2*2*2*5*5, 340=2*2*5*17
So the product of the shared primes is 2*2*5=20, so divide the numerator and denominator by 40.
(400/20)lbs/(340/20)ft^2
20lbs/17ft^2 or if you prefer
(20/17)lbs/ft^2
(1 3/17)lbs/ft^2 or approximately
1.18 lbs/ft^2
