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How many ways are there to arrange the letters $a, b, c,$ and $d$ such that $a$ is not followed immediately by $b?$

I am getting $6$ answer, but real answer is $18$

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    You might guess downvotes if you include [some context](http://meta.math.stackexchange.com/questions/9959/how-to-ask-a-good-question/9960#9960). For example, you could mention how you get the answer 6. Or what is the source of the problem (how do you know that the answer should be 18). – 2017-01-01

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Just subtract ways in which $a$ is followed by $b$ from total number of cases. $24-6=18$

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    ohh right ...tie man . Stupid question without thinking ask. Before ur answer it is not solve now solved . – 2017-01-01
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    Okay, you are welcome – 2017-01-01
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    Answer this also plz – 2017-01-01
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    In how many ways can a photographer at a wedding arrange six people in a row, including the bride and groom, if the bride is positioned somewhere to the left of the groom? – 2017-01-01
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    @sittian it's half the total number of arrangements. In the other half, she's somewhere to the right of the groom. – 2017-01-01
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    got it ..tie sir . – 2017-01-01
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I know that other answers give you a brief idea how it is happening but I hope you will also take a look here.

Since there are objects $a,b,c,d$, so the total ways of arranging them is $4!$. Now, we want the number of ways in which $a$ is not followed by $b$. So just take the cases when $a$ is followed by b and subtract them from total number of cases.

i.e., Fix $ab$ and you will see total arrangement in which $a$ is followed by $b$ is $3!$, (remember you cannot arrange $a$ and $b$).

So, Total ways become $4!-3!=24-6=18$

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    hey wolf below is ur meal .. plz solve it – 2017-01-01
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    In how many ways can a photographer at a wedding arrange six people in a row, including the bride and groom, if the bride is positioned somewhere to the left of the groom? – – 2017-01-01
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The total number of permutations is $= 4!$

The total number of permutations in which $a$ is followed by $b$ is $=3!$

The total number of ways in which $a$ is not followed by $b$ is $$= 4!-3!$$ $$=24-6$$ $$=18$$