I keep getting “Fundamental principle of counting” questions wrong in Permutations & Combinations. What's the trap?
adding when you should multiply independent choices.
The usual trap: adding when you should multiply independent choices. How to avoid it: 'and' usually means multiply; 'or' usually means add. Go back to the definition — if one task has m ways and another n ways, together they have m×n ways.
Key point
Fundamental principle of counting
Common mistake
adding when you should multiply independent choices.
Memory tip
'and' usually means multiply; 'or' usually means add.
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