book Maths Puzzle Numerical Ability

I can't seem to get the answer to this problem.

r=-0.08p^2+300p

Factor out the price on the right hand side of the formula

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book Other Question

inverse of g(x)=3^x+1
i get to the point of were i have the equation

x-1=3^y but i dont know how to get the y isolated on one side
May I ask another question please? In learning about a circle, the instructor explained that C=2 pi r, A=pi r squared. This should then give a formula for finding the radius and t