there are 9000 four digit numbers. 4536 of the four digit numbers have no repeating digits. How many four digit numbers have at
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We can use the Front Outside Inside Last (FOIL) method to expand the double bracket
Front ⇒
Outside ⇒
Inside ⇒
Last ⇒
Put the four terms together we have
, then collect like terms
Front ⇒
Outside ⇒
Inside ⇒
Last ⇒
Put the four terms together we have