The equation for this porblem is:
550 - x = 20 + 3x (Left side of equation is original length of pier minus shark piece. Right side is length of remaining dock plus 3 times the shark piece.)
subtract 20 from both sides:
520 - x = 3x
add x on both sides:
520 = 4x
divide by 4:
x = 130
So the piece the shark bit off was 130 ft.