Hutch Problem

Either solve $4r+2d=98$ and $r+d=35$ , either think this way:

There are 70 back legs of doves and rabbits. That leaves 28 front legs, which means 14 rabbits. Then there must be 21 doves.

Assume all are hares:

That means 35 x 4 = 140 legs

However, we know there are only 98 legs. As well as that, there's a difference of 2 legs from hares to doves.

140 - 98 = 42

42 / 2 = 21

Therefore, we know there are 21 doves.

Confirmation

14 hares x 4 legs = 56 legs

21 doves x 2 legs = 42 legs

42 + 56 = 98

Therefore, the answer is 21