To solve the problem, we can use Charle's law, which states that for an ideal gas at constant pressure the ratio between absolute temperature T and volume V remains constant:

For a gas transformation, this law can be rewritten as

(1)
where 1 and 2 label the initial and final conditions of the gas.
Before applying the law, we must convert the temperatures in Kelvin:


The initial volume of the gas is

, so if we re-arrange (1) we find the new volume of the gas:
Answer:
Object should be placed at a distance, u = 7.8 cm
Given:
focal length of convex lens, F = 16.5 cm
magnification, m = 1.90
Solution:
Magnification of lens, m = -
where
u = object distance
v = image distance
Now,
1.90 = 
v = - 1.90u
To calculate the object distance, u by lens maker formula given by:
u = 7.8 cm
Object should be placed at a distance of 7.8 cm on the axis of the lens to get virtual and enlarged image.