Rate of 1 machine * no of machines * time = 1 job

Rate of 1 machine *10 * 16 = 1

Rate of 1 machine = 1/160


using the rate of 1 machine above and the same formula

1/160*8*time = 1
time = 160/8 = 20

Another approach is:

no of machines*time = no of machines * time
10*16 = 8*x
x=20

i say B)20
i arrived at this answer by using an arbitrary value for the # of products being made.
let's say the machines are making pens, and since there are 10 machines and 16 hours, 160 pens seems like a good number to shoot for.
so if 10 machines work 16 hours to make 160 pens, then their combined per hour rate is 10 pens/hr. and, since there are ten machines, the hourly rate per machine is 1 pen/hr.
now, you're told that only 8 machines are making pens. the individual rate for each machine remains the same (1 pen/hr), so 8 machines each making 1 pen per hour means that the combined hourly rate is 8 pens/hr.
at this point, just apply the formula rate*time=distance, or in this case, r*t=pens. the 8 machines have to make 160 pens, and the rate of these 8 machines working together is 8 pens/hr, so the formula given these values is (8 pens/hr)(time)=160 pens...t=160/8, or 20 hours.
if you're looking for a purely algebraic explanation, well, you can derive it from everything i said . hope this helps

thank u ...u guys hv been a grt help