Let x be the total marbles (altogether). Write equation for total marbles+ + 12 = x. To solve, multiply all the terms by 8; then simplify 3x + 0.7*5x + 96 = 8x 3x + 3.5x + 96 = 8x 96 = 8x - 3x - 3.5x 96 = 1.5x x = 96/1.5 = 64. ANSWER. There are 64 marbles in the bag.