book Maths Puzzle Numerical Ability

a butcher has some hamburger that is 21% fat and some 15% fat. How many pounds of each should be mixed to make 84 lbs that is 17% fat?

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book Other Question

(2+3r+r^2)/(1+3r+3r^2+r^3)

Can you help me solve this? Thanks.
I have to find all the values of b in the following equation that will give one or more real number solutions. Can you please help me with this?

3x^2 + bx - 3 = 0

Thanks!