So first we know he walks 2 and 7/10 miles, which is the same as 2.7 miles.
We also know whenever you multiply a number by 10, all you have to do is move the decimal to the right ONE PLACE.
So here you would multiply 2.7 times 10. You move the decimal place right ONE place, so now you have 27.0. Therefore, he walked 27 blocks.
Hope that makes sense! If not feel free to ask any questions in comments! :D