Two amateur mathematicians were at UCLA to hear a lecture delivered by Terence Tao. One of them, Optimist Oliver said to his friend Pessimist Peter.
Oliver : I am going to impress him with my discovery. I have this number which has exactly factors and also has its last digits as ; and we all know that Terence Tao was born in .
Peter : You fool. You have made a mistake.
If you think Peter is right, what could be the correct number of factors of . If you think that Oliver is right, choose
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Optimistic Oliver must have made a calculation mistake.
For any number, N with prime factorization, N = p 1 i 1 p 2 i 2 ⋯ p n i n , the total number of factors can be calculated to be n f = ( i 1 + 1 ) ( i 2 + 1 ) ⋯ ( i n + 1 ) .
Now, analysing the last digits of N in the problem, we can see that the number ends in ⋯ 1 9 7 5 .
Now noting that for any a , 5 1 0 a + 5 = 2 a + 1 , and any number has to end in 5 or 0 to be divisible by 5 ; we can conclude that N divides 2 5 , but is not divisible by 1 2 5 .
5 N = ⋯ 3 9 5 , 2 5 N = ⋯ 7 9
From this, we get that N = 5 2 p 2 i 2 ⋯ p n i n and therefore the number of factors would be n f = 3 ( i 2 + 1 ) ⋯ ( i n + 1 ) .
The number of factors should be divisible by 3 . 1 9 7 5 is not divisible by 3 . Hence, Oliver is wrong.
Of the other two choices provided, 1 9 7 4 is divisible by 3