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Explanation:
We're told that "p is directly proportional to the square of q", which means that we have the equation
p = k*q^2
for some constant k
Replace p with 12 because we're also told that p = 12.
So we now have this equation
12 = k*q^2
which is the same as
kq^2 = 12
We'll use this later.
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Let's go back to the original equation. We'll replace q with 2q to represent the value of q doubling
p = k*q^2
p = k*(2q)^2 ... replace q with 2q
p = k*4q^2
p = 4k*q^2
p = 4*(kq^2)
p = 4(12) ...... replace kq^2 with 12
p = 48
Therefore, if q is doubled, then p = 12 quadruples to p = 48.
