Chapter 1 - Section 1.7 - Problem Solving: Interest, Mixture, Uniform Motion, Constant Rate Job Applications - 1.7 Asses Your Understanding - Page 141: 24

We have to mix 49 lbs. of the first coffee with 51 lbs. of the second one.

Let $x$ be the amount of the first coffee, then $100−x$ is the amount of the second coffee. Then our equation is: $2.75x+5(100−x)=100\cdot3.9\\2.75x+500−5x=390\\-2.25x=-110\\x\approx49$ Thus we have to mix 49 lbs. of the first coffee with 51 lbs. of the second one.