[phillyj]

When we react butane with Cl2 in the presence of light we get 1-chlorobutane and 2-chlorobutane in ratios of 30:70
Now 6 of the hydrogens in butane are primary while 4 are secondary in relation to the carbon. If we divide 30 by 6 we get that primary hydrogen contributed 5% of the product and 70/4= 17.5% so secondary hydrogen contributed 17.5% to the product.

my question is How do we know that the percent 1-chlorobutane should be divided by the number primary hydrogens and same for the 2-chlorobutane?