Question 1016979
0.2*x + 0.6*(100-x) = 0.2*100

0.2x + (60 - 0.6x) = 20

60 + 0.2x = 20 + 0.6x

40 = 0.4x

4x = 400

x = 100

You use 100kg of the 20% fat content chocolate, and none of the 60% fat content chocolate.