Explanation:
KI=yf/p+n + yr/u
(p+n) KI - yrp/u -yrn/u = yf
pKI - yrp/u = yf - nKI + yrn/u
p(KI - yr/u) = yf - nKI + yrn/u
p = (1/KI-yr/u)(yf-nKI+yrn/u)
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