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The expression $15^{80}$ x $28^{60}$ x $55^{70}$ gives a number that ends in a string of zeros.

How many consecutive zeros are in that final string?

I've done this type of question with factorials, but I've no idea how to approach this with indices. The given answer is 120, how is this achieved?

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It's the same trick, except easier. Pull out all the $2$'s and $5$'s to see how many $10$'s you get: $$ 15^{80}28^{60}55^{70} = 5^{80}4^{60}5^{70}(3^{80}7^{60}11^{70}) = 2^{120}5^{150}(3^{80}7^{60}11^{70}) = 10^{120}(5^{30}3^{80}7^{60}11^{70}) $$

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