Directly and Inversely Proportional Equations

Which equation exhibits a direct proportion?

• y = 5x - 2

• y = ³/₄ x

• y - x = 4

• x = 6

m is directly proportional to b

m=24 when b=6

Find the equation for m and b

• m = 4b

• m = 0.25b

• b = 4m

• 6m = 24b

g is directly proportional to b

g=20 when b=4

Find g when b=7

• 35

• 5

• 15

• 23

p is directly proportional to m

p=22 when m=2

Find the equation for p and m

• p = 11m

• p = 20m

• p = m + 20

• m = 11p

t is directly proportional to n²

t=144 when n=6

Find the equation for t and n

• t = 4n²

• t = 4n

• t = 24n

• t = 6n²

t is directly proportional to n²

t=144 when n=6

Find n when t=36

• 3

• 1.5

• 24

• 6

The cost of a circular table is directly proportional to the square of the radius.

A circular table with a radius of 40cm cost £50.

What is the cost of a circular table with a radius of 60cm?

• £33.33

• £75

• £112.50

• £125

t is inversely proportional to v

t=2 when v=6

Find the equation for t and v

• t = ¹²/_v

• t = ¹/₃ v

• t = ³/_v

• t = 12v

d is inversely proportional to j

d=6 when j=5

Find d when j=3

• 10

• 90

• 3.6

• 36

The time taken, t seconds, that it takes a water heater to boil water is inversely proportional to the power, p watts, of the water heater.

When P = 2000W, T = 252 seconds.

Find the time it takes to boil water when P = 800W

• 504000

• 630

• 320

• 504