Question:
I understand how series and parallel circuits are different and I
understand how to calculate the resistance [etc] in a parallel circuit,
but why is there LESS resistance when things are arranged in parallel [ie
two 10 ohm bulbs]?.

Replies:
Remember,
V = I x R
Think in terms of the current flow. For a given voltage, if the current
goes up the resistance must have gone down. If you provide one path the
current is restricted to the one path. If you provide a second, equivalent
path, twice as much current can flow. The current has increased, therefore the
resistance must have decreased. In essence, resistance measures how BAD
the path is for current flow. When there are two equivalent paths the
paths are LESS bad.

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