ACS Official Physical Chemistry Study Guide Question

I was going over the study guide, and ran across a problem that I can't find a solution to. Here it is:

A 10-L drum of ether at 18C is open to the atmosphere (P = 760 torr) and contains 6 L of liquid. The top is sealed and the drum dented so that it now has an 8-L capacity. If the vapor pressure of ether at 18C is 400 torr, what is the pressure inside the dented, sealed drum?

The answer is 1120 torr; however, I have no idea how they arrived at it.

If anyone has an suggestions, they would be appreciated.

Just consider this as a " simple " question ; with a few assumptions ( e.g. Volume of liquid ether remains at 6 L , No Temperature change when small amount of ether vaporize or condense ) .

At the beginning , when a 10-L drum containing 6 L of ether at 18C is open to the atmosphere ( P = 760 Torr )
Volume of gasses inside the drum = ( 10 L - 6 L ) = 4 L
Total pressure of gasses inside the drum = Atmospheric Pressure = 760 Torr
Partial pressure of ether = 400 Torr
Partial pressure of air = ( 760 Torr - 400 Torr ) = 360 Torr

Immediately , when the drum is sealed and dented to 8 L Volume
Volume of gasses inside the drum = ( 8 L - 6 L ) = 2 L
Total pressure of gasses inside the drum = 760 Torr x ( 4 L / 2 L ) = 1560 Torr
Partial pressure of ether should be 400 Torr x 2 - - > 800 Torr
Partial pressure of air should be = 360 Torr x 2 = 720 Torr

But some of the ether vapor will condensed into liquid ( to keep vapor pressure of ether at 400 Torr )

Then the final total pressure should be [ 400 Torr ( from ether ) + 720 Torr ( from air ) ] = 1120 Torr

Ans. 1120 Torr