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