pls help

3. A pneumatic valve is being used to control the flow-rate of a process liquid. The valve closes completely to prevent any liquid flow through it when the Input air pressure is 3 psig or below. The valve opens completely to allow a liquid flow rate of 30 LPM (Lits /min ) when the input air pressure is 15 psig or more. The dynamic response of the pneumatic valve to a step change in the input air pressure is expected to be 1st order. If the valve reaches 86.5% of its final flow-rate value, in 1min after a step change in the input air-pressure, find the valve transfer function (Find valve Gain=Kv and valve time constant v in minutes)