How many 6 digits telephone number can be formed with digits 0,1,2,3,4,5,6,7,8,9 if each number starts with 35 and no digits appears more than once.
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We have 6 places here to be filled
with 9 numbers, so that first two
digits from left should be 30. So fix 3
in 1st and 0 in 2nd place. So now we
have '4' places to be filled with 7
numbers without repetition.
We can do it in a simple way.
The third place has '7' chances, the
fourth one has '6', the fifth has '5',
the last one has '4'. So the total
number of chances are 7*6*5*4=840.
For easy calculation we can just use
formula.
'n'numbers should be placed in 'r'
places in 'nPr' ways.
Here 'n' is '7' and 'r' is '4'.
The answer is 7P4=840 ways!!!
Thanks
with 9 numbers, so that first two
digits from left should be 30. So fix 3
in 1st and 0 in 2nd place. So now we
have '4' places to be filled with 7
numbers without repetition.
We can do it in a simple way.
The third place has '7' chances, the
fourth one has '6', the fifth has '5',
the last one has '4'. So the total
number of chances are 7*6*5*4=840.
For easy calculation we can just use
formula.
'n'numbers should be placed in 'r'
places in 'nPr' ways.
Here 'n' is '7' and 'r' is '4'.
The answer is 7P4=840 ways!!!
Thanks
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