Dragons Alaranthus and Montal

The fully grown dragon Alaranthus has a weight equal to one-fifth of his own weight plus the weight of the dragon Montal, who weighs 800 pounds less than Alaranthus. How much does the mature Alaranthus weigh?

Answer:

4000 lbs.

Say a = Alaranthus's weight and m = Montal's weight.
Then
(1) a = m + (Vs)a
and
(2) a = m + 800.
Substituting m + 800 for a in equation (1) gives:
m + 800 ^ = m + ('/s)a; so 800 = (Vs)a, and a = 4000.