"I can add very fast," boasted Billy.  "Here, I'll show you. In this calendar, I want you to choose some full week of 7 days. It can be any wekk in any month as long as it has 7 days.  Now add the numbers in that week.  But don't tell me the sum.  Just tell me the number of the first day of the week, and I'll find the sum, too."
	Mary chose the week of April 8.  Shee added the numbers very carefully. (Did she get 77?)
	"I'm ready," said Mary.   "I chose the week in April beginning on the eighth."
	"Then your sum is 77," said Billy after a breif moment.
	"That's right, Billy.  How did you add it so fast?"

The key is add 3 to the first day of the week.  Then multiply the sum by 7. 

Think about why this works and where else you can use it.