Chapter 1 - Section 1.7 - Problem Solving: Interest, Mixture, Uniform Motion, Constant Rate Job Applications - 1.7 Asses Your Understanding - Page 141: 23

See below.

Let $x$ be the amount of Earl Grey Tea, then $100-x$ is the amount of Orange Pekoe Tea. Then our equation is: $5x+3(100-x)=100\cdot4.5\\5x+300-3x=450\\2x=150\\x=75$ Thus we have to mix 75 lbs. of Early Grey Tea with 25 lbs. of Orange Pekoe Tea.