There are 4 weeks in a month so Cathy and Chris were supposed to work for 4 weeks/month x 2 months = <<4*2=8>>8 weeks each.
Since Chris got sick then Chris was only able to work for 8 weeks - 1 week = 7 weeks while Cathy worked for 8 weeks + 1 week = 9 weeks.
Therefore, Cathy worked for 9 weeks x 20 hours/week = <<9*20=180>>180 hours.
#### 180