Explanation:
x = pounds of Tanzanian coffee
y = pounds of Terraza coffee
x + y = 40
x = 40 - y
x×10.20 + y×13.60 = 40×12.58 = 503.20
(40 - y)×10.20 + y×13.60 = 503.20
408 - y×10.20 + y×13.60 = 503.20
y(13.60 - 10.20) = 95.2
y×3.40 = 95.2
y = 95.2 / 3.4 = 28
x = 40 - 28 = 12
he needs to mix
12 pounds of Tanzanian coffee with 28 pounds of Terraza coffee.