let

x = lbs of 20% fat-content chocolate

y = lbs of 15% fat-content chocolate

candy company mixes a 20% fat-content chocolate with a 15% fat -content chocolate to obtain 50 pounds of a 16% fat-content chocolate

x + y = 50

0.20x + 0.15y = 0.16*50

by solving the system of equations we find

x = 10 lbs

y = 40 lbs

click here to see the step by step solution of the system of equations

10 pounds of the 20% fat-content chocolate and 40 pounds of the 15% fat-content chocolate should be used.