John is going to work. it takes him 25 mins to walk to the bus stop, 45 minutes on the bus, then 10 minutes to walk from the bus stop into work. at what time should he leave the house in order to get to work at 9: 00 a. m.?

Add up all of the numbers! 25+45+10=
Then subtract from 9:00 o'clock

Time to walk = 25 mins,  45 mins on bus and 10 mins walk.
Total = 25 + 45 + 10 = 80 mins =  1 hour 20 minutes.

So we need to subtract this from 9:00 am

9 : 00  -  1: 20 =  7: 40

So he has to leave the house at 7:40 am.
And he shouldn't do any other thing, but to follow the schedule in the question.
Like he shouldn't stop for a chat or snack.
Cheers.