Heating kills most of the bacteria in the solution. Only some bacteria will survive whose temperature optima is higher, or are heat resistant.
Formula for calculating CFU/ml = no. of colonies * dilution factor / volume plated (in ml)
Suppose the number of colonies in normal solution plate comes out to be 300 and that in the heated solution plate comes to be 30, then cfu/ml for heated solution = 30*10^0/ 0.1 = 300 cfu/ml; and cfu/ml for normal solution = 3000cfu/ml
