Achocolate shop makes a dark chocolate that is 38% fat and a white chocolate that is 48% fat. how many kilograms of dark chocolate should be mixed with 50 kg of white chocolate to make a ripple blend that is 40% fat?

Let x be number of kilograms of dark chocolate in the ripple blend. Then,

Total fat in the chocolates = 38*x+48*50 =38x+2400
Total fat in the blend = 40(x+50) = 40x+2000

The two equations are equal and thus,
38x+2400 = 40x + 2000 => (40-38)x = 2400-2000 => 2x = 400 => x = 400/2 = 200 kg