There are 2 dog walkers and they each have 2 legs so that's 2*2 = <<2*2=4>>4 legs
The other dog walker is walking 3 dogs that each have 4 legs so that's 3*4 = <<3*4=12>>12 legs
The dog walkers have 4 legs and the other dogs have 12 legs for a total of 4+12 = <<4+12=16>>16 legs
There are 36 legs total and 16 belong to the dog walkers and 3 dogs so that leaves 36-16 = <<36-16=20>>20 legs
Dogs have 4 legs and there are 20 legs so Mariel is walking 20/4 = 5 dogs
#### 5