Window Washing

When Mr.Watson built his home, he made one wall of the living room entirely out of thick plate glass. He soon realized how expensive as well as inconvenient it was to live in a "glass fishbowl", for the glass had to be washed on the average of once a week. It was too big a job for him, since the wall measured nineteen by twenty feet, so he contracted with a window cleaning service to do the full job once a week at the cost of one dollar per hundred square feet of surface washed, what was Watson weekly bill?

SOLUTION :

The "full job" would have to include both the inside and outside 2 times 19 times 20 equals 760 square feet of surface to be washed. The bill was thus $7.6 per week. -QED-