oh,it is not mathematical problem,but you are great !
I just wanna that the 60 sound and 120 sound ,the difference between them ,it depends on hub structure inside of hub gear wheel. 60 sounds , the space between gear wheel is twice double against 120 sound that helps cyclist to save more energy,may fell more comfortable when cycling. more hug gear wheel ,smaller space between gear wheel,the rate of trackslip of hub is more than 60 sound .