1.N is a 50 digit no.All the digits except the 26th from the right are 1.If N is divisible by 13 then the unknown digit is?
21I think you will not need the complete solution. Note that 111111 is a multiple of 13. So you just have to rewrite the given number in such a way that it becomes a multiple of 13 + something. Now since the number is divisible by 13 , this "something" must also be divisible by 13.
Try this. The answer will be 3.
1starting off where shobhodip left.
N is
11111100000..... (44 zeroes)
+11111100000.... (38 zeroes)
+11111100000.... (32 zeroes)
+11111100000.... (26 zeroes)
+a100000.... (24 zeroes)
+11111100000.... (18 zeroes)
+11111100000.... (12 zeroes)
+11111100 (6 zeroes)
+111111
so a100000.... (24 zeroes) should leave remainder 0 when divided by 13
hence a is 9
