direct proportion

y =kx 

y is directly proportional to x

k is the constant of proportionality number

example

a) write an algebraic formula for y in terms of x                                                                                    y = kx

b) when x =4 and y=10 find the constant of proportionality

1.write it out with the numbers 10 = 4k

2. move it round so k is the subject of the formula k = 10 / 4 = 2.5

3.put it back in the formua   y =2.5x