Let x be the number of kilograms of 10% fat content chocolate and y be the number of kilograms of 50%, then we can set the following system of equations:
Solving the first equation for x we get:
Substituting x=100-y in the second equation and solving for y we get:
Substituting y=70 in the third equation we get:
Answer: They must use 30 kilograms of 10% fat content chocolate and 70 kilograms of 50% fat content chocolate.