let abby has a candies, billy has b,charlie has c and david has d
c=2b => c=4a
now all equation are depending only on the value of a .
it is given that value of a <4
now apply hit and trial for a=1,2 and 3 .
Abby had 2 pieces,
Billy had 4 pieces,
Charlie had 8 pieces,
and David had 4 pieces.