In a study of mutation in fruit flies, researchers radiate flies with X-rays and determine that the mutation percentage M increases linearly with the X-ray dosage D, measured in kilo-Roentgens (kR). When a dose of D = 3 kR is used, the percentage of mutations is 7.7%, while a dose of 5 kR results in a 12.7% mutation percentage. Express M as a function of D. What percentage of the flies will mutate even if no radiation is used?