Multiple of 10

Which of the following is a multiple of 10?

A . 3 7 100 3 7 20 B . 3 7 500 3 7 4 C . 3 7 4 1 D . 3 7 500 1 A. \hspace{5mm} 37^{100} -37^{20} \\ B. \hspace{5mm} 37^{500} - 37^{4} \\ C. \hspace{5mm} 37^{4} - 1 \\ D. \hspace{5mm} 37^{500} - 1

All of them A and B only None of them C only A, C and D only

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3 solutions

All numbers end with 7 7 and have power of multiple of 4 4 , so they will have last digits as 1 1 . On subtracting them the last digits of all of them will be 1 1 = 0 1-1=0 , so all are multiples of 10 10 .

James Watson
Aug 15, 2020

We know that if the power of any number that ends in 7 is divisible by 4 4 , it ends in a 1 1 . We can use this to our advantage.

  • Let us look at C first. 4 0 ( m o d 4 ) 4 \equiv 0 \; \; \pmod{4} , so the number ends in a 1 1 . since we subtract 1 1 from it, it must end in a 0 0 and is therefore divisible by 10 10 !
  • Next, onto D. 500 0 ( m o d 4 ) 500 \equiv 0 \; \; \pmod{4} and the same principle can be applied here from C, so it is divisible by 10 10 !

Now that we have cleared up C and D, we can move onto A and B.

  • For B, we know that 3 7 500 37^{500} and 3 7 4 37^4 both end in a 1 1 . Since the last digits will subtract to make 0 0 , it is divisible by 10 10 !
  • Finally, for A, we must see if these both end in 1 1 so that we can draw the same conclusion as B. 100 0 ( m o d 4 ) 100 \equiv 0 \; \; \pmod{4} and 20 0 ( m o d 4 ) 20 \equiv 0 \; \; \pmod{4} so they both end in a 1 1 and by using the same reasoning as B, it is divisible by 10 10 !

So overall, A, B, C and D \green{\boxed{\text{A, B, C and D}}} are all multiples of 10 10 !

Barry Leung
Aug 15, 2020

Use Euler's Theorem and you will find all the values are equivalent to 0 (mod 10).

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