Let E and C be the number of stamps Eduardo and Carson had.
E=C+16. C/2=E/4, so multiplying through by 4: 2C=E.
Now put E=2C in the other equation: 2C=C+16, so C=16 and E=C+16=32.
Eduardo gives x stamps away, so now he has 32-x stamps. 32-x=3/8C=3/8*16=6 stamps. 32-x=6 so x=32-6=26. Eduardo gave 26 stamps away.