If it takes 1 min (max) to screw in a lightbulb, and 2 people would make the job in 30 seconds. After 30 mins, why haven't my parents left my sisters room?

Math

var P1= 1m

var P2= 1m

var : [(P1+P2)/2]/2 = 30seconds

if lightbulb = 30 seconds

why lightbulb = 30+ minutes

var str: also how do you know if you house is haunted?