How many 7s in 555756756775775565556 are immediately preceded by 6 but NOT immediately followed by 5

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lemjay | High School Teacher | (Level 3) Senior Educator

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555756756775775565556

There are a total of six 7's in the given number. But we need to determine the number of 7's that satisfy the condition that the 7 is preceded by 6 but NOT followed by 5.

Starting from the left, let's try to check the number before and after 7.

_ _ 575_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>The 7 here is preceded by 5 and followed by 5, so this does not satisfy the condition above.

_ _ _ _67_ _ _ _ _ _ _ _ _ _ _

> The 7 is preceded by 6 and followed by 5. Still, this does not satisfy the given condition.

_ _ _ _ _ _ 677_ _ _ _ _ _ _ _ _ _

 

> The 7 is preceded by 6 and followed by 7. Hence this satisfy the condition the number before7should be 6 and after it ,should not be 5.

 

 

 

_ _ _ _ _ _ _ _ _ _775_ _ _ _ _ _

> The 7 is preceded by  7 and  by 5. Again, this does not satisfy the above condition.

Hence, the number of 7's that is preceded by 6 and not followed by 5 is one only.

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