ARITHMETIC OF MILK AND MILK PRODUCTS 203 



Applying Rule I, we find the weight of fat in each 

 lot of milk, the results being indicated in the third 

 column above. The total amount of fat in all of the 

 milks is 72.1 pounds, which, divided by i,6oo (the 

 total weight of milk), gives 4.5 as the real average 

 per cent, of fat in all the milk. 



It is wrong to regard as the average per cent, the 

 result obtained by adding the per cents, directly and 

 then dividing this sum by the number of lots repre- 

 sented, unless the amounts of milk or milk products 

 are equal. Thus, in the foregoing example, the result 

 of such a wrong method would make the average 4 

 per cent., when it is really 4.5. 



The same principle explains why we do not get a 

 true average composite sample, when we take the 

 same amount of milk from different lots that vary 

 considerably in weight and per cent, of fat. 



EXAMPLES FOR PRACTICE 



(i) Find the average per cent, of fat in the follow- 

 ing lots of milk: 1,200 pounds, 3 per cent, of fat; 

 2,000 pounds, 5 per cent, of fat; 6,000 pounds, 4 per 

 cent, of fat; and 1,800 pounds, 3.5 per cent, of fat. 



(2) Find the average per cent, of fat in 1,000 

 pounds of cream, 40 per cent, of fat; 1,600 pounds, 30 

 per cent, of fat; and 400 pounds, 20 per cent, of fat. 



25. FINDING AMOUNT OF CREAM 



Rule. — To find the amount of cream produced for 

 100 pounds of milk when the per cent, of fat in milk 

 and in cream is known, divide the per cent, of fat in 



