EXAMPLE 1 Calculating an index number

A gallon of unleaded regular gasoline cost $0.992 during the first week of January 1994 and $2.689 during the first week of August 2015. (These are national average prices calculated by the U.S. Department of Energy.) The gasoline price index number for the first week in August 2015, with the first week in January 1994 as the base period, is

The gasoline price index number for the base period, January 1994, is