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P = (2pt) / (t + 4)
and assuming that P and p are different variables, multiply both sides by t + 4
P(t + 4) = 2pt
Pt + 4P = 2pt
now collect terms with t in them
Pt - 2pt = -4P
t(P - 2p) = -4P
t = -4p / (P - 2p)
If P and p are meant to be the same, then
t = 4