Robin has triple as many marbles amy. sam has 15 more marbles than robin. together they all have 99 marbles. how many marbles does each person have

Let:     x be Amy's marbles
         3x be Robin's marbles
   15+3x be Sam's marbles
x+3x+15+3x=99
          7x+15=99
                7x=99-15 (transfer 15 to the right side of the equation, then minus)
             7x/7=84/7 (divide both sides of the equation by 7)
                  x=12
Now substitute:
                             x = 12 (Amy's)
                3x = 3*12 = 36 (Robin's)
   15+3x  = 15+3*12 =  51 (Sam's)
               12+36+51 = 99
Therefore, Amy has 12 marbles, Robin has 36 marbles and Sam has 51 marbles.

Answer might be d.i think