average calculation with hundredths of seconds?

Hi guys i'm trying to get the average of two times. In other words I have two times in minutes:seconds:hundredths_of_seconds and i'm trying to get the average of these times but i just cant seem to get the hundredths of seconds right.

for example if i have 00:02:20 and 00:04:40 (minutes:seconds:hundredths) it should return 00:03:30
i was able to get the minutes and the seconds just fine but i am going desperately crazy in trying to get the hundredths of seconds right.

here is the code:

def lap_average(lap1, lap2): 
	
	mins1, secs1, hundred_sec1 = lap1.split(":") 
	mins2, secs2, hundred_sec2 = lap2.split(":")   
	 
	minutes = int(mins1) + int(mins2)
	seconds = float(secs1) + float(secs2)
	h_seconds = float(hundred_sec1) + float(hundred_sec2)
	hundredths = int(60000 * minutes + 1000 * seconds + 0.001 * h_seconds)
	hundredths = hundredths // 2
	
	secs, hundredths = divmod(hundredths, 1000)
	minutes, secs = divmod(secs, 60)
	hundredths = ((hundredths/1000)*1000)

	print minutes, secs, hundredths

# testing times
lap_average('03:40:00', '05:20:00') 
lap_average('03:00:02', '02:00:00') 
lap_average('02:25:50', '06:50:75') 
lap_average('00:02:00', '00:03:00') 
lap_average('00:02:20', '00:04:40') 
lap_average('02:40:40', '03:30:30') 
lap_average('02:60:30', '60:40:40')

Any help in achieving this would be much appreciated.

P.S - please be nice i'm a newbie