If a restaurant bills totals $85, what amount should be left as a 15$ tips
a. $97.75
b. $11.90
c.$12.25
d. $12.75

Atourist boat is used for sightseeing in a nearby river. the boat travels 2.4 miles downstream and in the same amount of time, it travels 1.8 miles upstream. if the boat travels at an average speed of 21 miles per hour in the still water, find the current of the river.