Two methods:

1. Without using algebra:

196 legs is the same as half that number of leg pairs, or 98 leg pairs.  Hens
and rabbits both have 1 front leg pair each.  Only rabbits have 1 back leg pair
each.  There are 70 heads, so there are 70 animals.  Those 70 animals each have
1 pair of front legs. So 70 of the 98 total leg pairs are front leg pairs, and
the remaining 28 are back leg pairs, so there are 28 rabbits and 
 70-28 or 42 hens.

2. With algebra:

 h +  r =  70
2h + 4r = 196   

Solve that system and get h = 42, r = 28

Edwin