Hello!
A certain lock has five buttons, numbered from 1 to 5. The
combination of numbers that opens the lock is a sequence of seven
numbers and is reset every week.
(a) How many combinations are possible if every button must be
used at least once?
(b) Assume that the lock may also have combinations requiring to
push rst two buttons simultaneously and then the other ve
one at a time (still with the restriction that every button must
be used). How many combinations does this add?
So i managed to solve A) the answer is 16800. but i stuck at b)
Any hints?
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