Inverse
Eduardo counted ten seconds between seeing lightning and hearing thunder, and he knew that the lightning was about 2 miles away. If he counted four seconds between the next flash of lightning and thunder, about how far away was the lightning?
0.8
The formula for direct variation
y=kx or y/x=k
On a map, distance in km and distance in cm varies directly, and 25 km are represented by 2cm. If two cities are 7cm apart on the map, what is the actual distance between them?
87.5 km
The formula for inverse variation
y=k/x or xy=k
If y varies inversely as x, and y=2 when x=8, find y when x=32.
1/2
If y varies directly as x, and y=16 when x=4, find y when x=12.
48
If y varies directly as x, and y=3 when x=9, find x when y=5.
15
If y varies inversely as x, and y=7 when x=4, find y when x=28.
1
The time taken to cycle a particular distance varies inversely with the speed of the bicycle. Tim takes 3 hours to reach his destination traveling at a constant speed of 12 miles per hour. Find the constant of proportionality.
36
If x varies inversely as y, and x=16 when y=5, find x when y=2.
40
The time it takes to fly from Los Angeles to New York varies inversely as the speed of the plane. If the trip takes 6 hours at 900 km/h, how long would it take at 800 km/h?
6.75
If y varies directly as x, and y=375 when x=5, find y when x=3.
225
Disregarding wind resistance, the distance a body falls from rest varies directly as the square of the time it falls. If a skydiver falls 64 ft in 2 seconds, how far will he fall in 10 seconds?
1600
In planning a trip, the time it takes to make the trip varies inversely with the rate of the car. If I take Route A and average 40 mph, it will take 9.5 hours to complete the trip. If I go by Route B and average 50 mph, how long will the trip take?
7.6