Combined Variation

It is given that m varies directly as the square of n and inversely as p. Given m=10.8 when n=18 and p=6. Express m in terms of n and p.

y varies directly as x and inversely as z. It is given that y = 51 when x = 1.7 and z = 5. Express y in terms of x and z.

 $It\ is\ given\ that\ y\propto\frac{\sqrt{x}}{z}and\ y=32\ when\ x=2.56\ and\ z=0.4.$  Find the value of y when x = 81 and z = 6.

It is given that 

The time taken T minutes, to boil water varies directly as the weight, w kg, of the water and inversely as the power, P watts, of a kettle. Shamsiah uses 4 minutes to boil 2 kg water using a kettlw with the power of 1 500 watts. (T = 4, w = 2, P = 1 500)

(I) Express T in terms of w and P. (Find k and then express...)

(ii) If the power of the kettle used is 1 200 watts, what is the time needed, in minutes, to boil 1.8 kg water? (Find T, when P = 1 200, w = 1.8)