QUESTION

CSAT

Easy

Maths

Prelims 2023

A number N is formed by writing 9 for 99 times. What is the remainder if N is divided by 13?

Select an option to attempt

Explanation

Method I:

N = 9999999999 \ldots 99 times

Any digit repeated (P1)(P - 1) times is divisible by PP, where PP is a prime number >5> 5.

So, 99999999 \ldotsrepeated 131=1213 - 1 = 12 times will be divisible by 1313.

So, 99999999 \ldotsrepeated 12×8=9612 \times 8 = 96 times will be divisible by 1313.

That is, Remainder [999996 times /13]=0[9999 \ldots 96 \text{ times } / 13] = 0

Or Remainder [9999(96 times)000/13]=0[9999 \ldots (96 \text{ times}) 000 / 13] = 0

So, we just need to find out the remainder when we divide the remaining three digits by 1313.

Remainder [999/13]=11[999 / 13] = 11

Hence, option (a) is correct.

Method II:

We can analyze the pattern of remainders. Remainder [9/13]=9[9/13] = 9 Remainder [99/13]=8[99/13] = 8 Remainder [999/13]=11[999/13] = 11 Remainder [9999/13]=2[9999/13] = 2 Remainder [99999/13]=3[99999/13] = 3 Remainder [999999/13]=0[999999/13] = 0 This pattern can be seen getting repeated thereafter too. Remainder [9999999/13]=9[9999999/13] = 9 Remainder [99999999/13]=8[99999999/13] = 8 ...and so forth.

So, if total number of 9s is six, twelve, eighteen, ......., ninety, ninety six, etc., the remainder is 0. So, if the number has ninety seven 9s, the remainder is 9. [Following the pattern] So, if the number has ninety eight 9s, the remainder is 8. So, if the number has ninety nine 9s, the remainder is 11.

Therefore, the answer is 11.

Hence, option (A) is correct.

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