Answer:
Joint variation states that:
If z is directly proportional to x and inversely proportional to y,
i.e,
, 
then the equation is in the form:
where k is the constant of variation.
From the given information: The volume V of a gas varies inversely as the pressure P and directly as the temperature
i.,e
, 
by definition of joint variation:
.....[1] where k is the constant of variation.
It is also given that a certain gas has a volume of 10 liters(L) , a temperature of 300 kelvins(K), and a pressure of 1.5 atmosphere(atm).
Substitute these given values in [1] to solve for k;

Simplify:

Divide by 200 both sides we have;

Now, if a gas has a temperature of 400 kelvins and a pressure of 5 atm.
To find the volume (V);
Now substitute the given data and value of k in [1] we have;

Therefore, the Volume of a gas is, 4 liters