Remember that to calculate resistances in parallel, the formula we use is:
1/Rt = 1/R1 + 1/R2 etc.
So that gives us:
1/Rt = 1/300 + 1/250 = 0.0073
To find the total resistance we just take the reciprocal of this:
Rt = 1/0.0073 = 136.4 Ohms
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