Total resistance is R+r, so
R/(R+r) of the power is delivered to the load.
A good worksheet on this is to be found at
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/autbat.html#c2
An electrical source with internal reistance r is used to operate a lamp of resistance R. What fraction of the total power is delivered to the lamp?(a)R+r/R(b)R-r/R (c)R/R+r (d)R/r
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