Interval estimates question

Hi Guys,

Just discovered this forum today, it seems like a great resource for stats people.

I'm relatively new to the stats world (I've done a couple of units at University) and need some help understanding something.

I do have a particular question I'm trying to answer but I would rather understand the intuition rather than just get the answer.

Supposing I have a fitted equation which tests the time it takes Sam to get to work. There are 3 explanatory variables in the equation which obviously try to predict the time it takes for him to get to work.

But the question asks for the interval estimate where there is a set condition. I.e. if we say that the number of streets he takes to work is an explanatory/independent variable (it's not but for the sake of discussing it let's say it is) and the amount of Police stations he passes is also a factor, as well as the number of bridges he crosses.

The equation would be something like TIME = B1 (intercept) + XNOSTREETS + XPOLICE + XBRIDGE + e

How do we find the interval estimates given he uses 7 streets, 2 police stations and 1 bridge at the 10% level?

What's the logic behind this too? THanks guys.