The mileage cost for the first apartment will be 31*20*0.58 = $<<31*20*0.58=359.60>>359.60
This makes the total monthly cost of the first apartment 359.60 + 800 + 260 = $<<359.60+800+260=1419.60>>1419.60
Similarly, the mileage cost for the second apartment will be 21*20*0.58 = $<<21*20*0.58=243.60>>243.60
Thus, the total monthly cost of the second apartment is 243.60 + 900 + 200 = <<243.60+900+200=1343.60>>1343.60
Therefore, the difference in total monthly costs is 1419.60 - 1343.60 = $<<1419.60-1343.60=76>>76
#### 76