%ld?

Answer 1

%ld is a format specifier for long int.
When an integer is too long , a long int format specifier is used.😀

Answer 2

Be more descriptive when asking a question.
Reference helps
http://www.cplusplus.com/reference/cstdio/scanf/
http://www.cplusplus.com/reference/cstdio/printf/
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Answer 3

It means "Long integer".
For instance, if integer is 2 bytes, long integer is 4 bytes. It allows to you to use longer values