let no. of units of A wihch are sold = a , where 100a300
and no. of units of B which are sold = b , where 100b250
so profit p = 5a+4b................(1)
since labour hours are fixed so we have 10a+8b=4000 or 5a+4b = 2000........(2)
since from (2) 5a+4b (which is also equals to profit ) has a fixed value independent of a and b
so profit will be fixed independent of a and b
if we choose a such that it lies between 100 and 300 then we can find b using (2).
for example if we choose a= 200 then b will be 250 which is valid.
here we have to take care that values of a and b lies within the range specified.