Answer
a) 1.46 Ns
b) 27.04 kilograms
Work Step by Step
a) We know that the impulse is equal to the change in momentum. After hitting the rhino, the bullet has no momentum, so we find:
$ J = m\Delta v = .02 \times 73 = \fbox{1.46 Ns}$
b) We know that the momentum of all of the bullets adds to the initial momentum of the rhino. Thus:
$.81m = .02 \times 73 \times 15 \\ \fbox{m=27.04 kilograms}$
