​ What is the total number of miles Pablo runs over the course of 12 days?

The sum is given by

[ distance on day 1 + distance on day 12 ]  *  12  / 2

The distance on day 12 is   

1.5  + .5 (11)   =   7

So...we have

[ 1.5  + 7 ] * 12  / 2  =

[ 8.5 ] * 6  =

51  miles