Cool story. I think you might be asking how long they had to walk and ride.
3.5*.66 = 2.31 miles drove
3.5*.33 = 1.15 miles walked
To answer this you need to divide the total by 4 so 28 divided by four is 7 and 7*3=21 so Sam has 21 pencil erasers
sea level is 0 and a deck is above water, so 10 and below sea level is where people scuba dive, so the dot at -15
2.6 would be the proper answer to this question