Answer:
4
Step-by-step explanation:
1. if the mass of coffee worth $10 is 'x', then its worth is 10x, and the worth of the mixture is 5(x+20);
2. using the previous item it is possible to write the next equation:
5(x+20)=4*20+10*x; ⇒ x=4.
3. the required mass is 4 pounds.
