The volume v of a fixed amount of a gas varies directly as the temperature t and inversely as the pressure p . suppose that v=160cm3 when t=320 kelvin and p=12kg/cm2 . find the pressure when t=240 kelvin and v=90cm3 .

For this question, we can use the general gas law. Where (p1xV1/T1) = (p2xV2/T2)
In that equation, "1" means the initial value, and "2" is the new value given. 
p is the pressure, V is volume, and T is the temperature in Kelvin (if the temperature given is not in Kelvin, change it before you calculate!)

Now, simply substitute the numbers in, and we can calculate the final pressure, p2. 

12 x 160 / 320 = p2 x 90 / 240
6 = p2 x 90 / 240
p2 x 90 = 1440
p2 = 16

Therefore, your answer should be 16kg/cm2