Rent from previous tentant

So, I moved in with a guy on august 21st signed the lease and paid rent for September-December( I was never told I was responsible for august). I had assume that the guy I as moving in with had paid the rent, but he had only paid half. Today I got an eviction notice saying I was late on rent for august.
Is this legal?
Why wasn't I told about this 4 months ago?
Am I responsible for my now roommate not paying the full rent in august?
Did the leasing office have any responsibility to tell me of the unpaid portion of the rent when I signed the lease?

Thank you,
Washington State, Whitman county.

[OHlandlord]

When more than one tenant is on a lease, they become legally "jointly and severally liable" for ALL of the obligations under the lease. That means that both tenants are completely liable for the full rent amount. If your roommate failed to pay his share, then you are responsible for it all. You will owe for the rent your roommate failed to pay for the period when you lived there.

I would contact the leasing office and let them know that you didn't sign or move in until the 21st of August. As such, you were only in the unit, and on the lease, for 1/3 of that month and should only owe 1/3 of the rent for the month. Offer to pay that amount immediately along with any late fee that is written into the lease. You should not owe rent for any period before you moved into the unit.

You may not have been notified because the leasing office was applying current rents paid to the past due balance. So when you paid for September, they applied it to August; etc.