HOW TO PURCHASE PRE FORECLOSURE / IS THIS A DEAL?

I came across a pre-foreclosure property which i am interested in purchasing as a subject to. Owner is behind in payments by $20,000. there is a mortgage on it for $90,000. Current mortgage is $780/month (PITI). the ARV is $140,000, it is a 2bed 1bath home in a great neighborhood. if i were to acquire this property and rent it out for $1200, it can produce a cash flow of about 420/month. My concern is the $20,000 in arrears, is there anyway around paying it? how would you approach this one?