How can I run below script file in SQL*Plus.
Script file content:
variable a number;
begin
:a := 10;
end;
print a;
How can I execute this in SQL*Plus?
EDIT
Script file name is: Question3.sql
I am getting following error:

In order to execute a PL/SQL block in SQL*PLUS, you have to terminate it with the slash /. So, your file should look like this:
-- some_name.sql file
variable a number;
begin
:a := 10;
end;
/
print a;
SQL> @c:\some_name.sql
PL/SQL procedure successfully completed.
A
----------
10
You could also use exec SQL*PLUS command to execute a single line command - assign a value to a bind variable in your case. exec command implicitly wraps a statement you are trying to execute in the begin end block:
-- some_name.sql file
variable a number;
exec :a := 10;
print a;
SQL> @c:\some_name.sql
PL/SQL procedure successfully completed.
A
----------
10