just plug and chug:
1/7 = 1/r2 + 1/15
1/7 - 1/15 = 1/r2
8/105 = 1/r2
r2 = 105/8 = 13.125
A resistor has a resistance of 15 ohms. What size of resistor, (r2), should be connected in parallel with it to obtain a resistance of 7 ohms.
Use 1/R=1/r1+1/r2.
r2= ohms?
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