When you measure a mass on a balance, typically you measure the total and the tare:

Sample = Total - Tare

if the uncertainties are given by normal distributions and are not correlated

Sample = Total - Tare
Sample = Total + Tare

A good way to measure electrical power: multiply the current and the voltage drop

Power=I*V
Power = I v

Power = Power [(I/I)2 + (v/v)2]1/2

Division works like multiplication. More complicated operations give very complex error propagation: a topic of ongoing research.