Zeroes everywhere

Find the number of trailing zeros of the number 60 ! 60! .

10 14 15 16

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2 solutions

Eamon Gupta
Jun 15, 2015

Since the factors 5 and 2 are needed to make a trailing zero, we can take the floor function of how many times the powers of 5 divide 60 (since they are less common than powers of 2).

60 5 + 60 5 2 = 12 + 2 = 14 \left \lfloor \frac{60}{5} \right \rfloor + \left \lfloor \frac{60}{5^{2}} \right \rfloor = 12+2 =\boxed{14}

In general, the number of trailing zeroes in x! = x 5 + x 5 2 + x 5 3 . . . + x 5 n w h e r e x 5 n + 1 < 1 < x 5 n \left \lfloor \frac{x}{5} \right \rfloor + \left \lfloor \frac{x}{5^{2}} \right \rfloor + \left \lfloor \frac{x}{5^{3}} \right \rfloor ... + \left \lfloor \frac{x}{5^{n}} \right \rfloor where \frac{x}{5^{n+1}} < 1 < \frac{x}{5^n}

NB: x \left \lfloor x \right \rfloor is the floor function of x and is used to denote the largest integer not greater than x

row 1 col 1 row 1 col 2
row 2 col 1 row 2 col 2

That is not a solution

Oon Han - 3 years, 3 months ago

what is this?

Abha Vishwakarma - 2 years, 8 months ago

what the hell is this?

Kshitiz Poudel - 11 months, 1 week ago

What the hell does this mean???

Lâm Lê - 9 months, 2 weeks ago

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