Answer:
Stephen weighs 80 lbs
Graham weighs 60 lbs
Step-by-step explanation:
Let Stephen the cat = S
Let Graham the cat = G
25% heavier = 125% = 125/100 = 1.25
50% heavier = 150% = 150/100 = 1.5
If both animals gained 20 lbs, S would be 25% heavier than G:
  S + 20 = 1.25(G + 20) Â
⇒      S = 1.25(G + 20) - 20
⇒      S = 1.25G + 5
If both animals lost 20 lbs, S would be 50% heavier than G:
  S - 20 = 1.5(G - 20) Â
⇒      S = 1.5(G - 20) + 20
⇒      S = 1.5G - 10
Equate both equations for S, and solve for G:
         S = S
⇒ 1.25G + 5 = 1.5G - 10
⇒   -0.25G = -15
⇒        G = 60
Substitute found value for G into one of the equations for S and solve for S:
  S = 1.5G - 10 Â
⇒  S = 1.5 x 60 - 10
⇒  S = 90 - 10
⇒  S = 80
Stephen weighs 80 lbs
Graham weighs 60 lbs