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secondary 3 | A Maths
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Hi, I am stuck with these questions. Can't solve any of them. Kindly advise the working solutions. Thanks a lot!
For the first one, it’s very straightforward. Factorials (with exclamation sign) means the product of all whole numbers up to that number, so 5! means 1 x 2 x 3 x 4 x 5. This is 120. Multiplying by future numbers do not change the last digit “0”, so 2022! will have a last digit of zero.
If I am able to come up with a solution, I will post them.
In short, we have to find
1. Numbers which are multiples of 625; these will contain four “5”s per number
2. Numbers which are multiples of 125 but not 625; these will contain three “5”s per number
3. Numbers which are multiples of 25 but not 125; these will contain two “5”s per number
4. Finally, numbers which are multiples of 5 but not 25; these will contain one “5” per number
For the 2s, there are simply way too many of them, because within the first 10 numbers, 2, 4, 6, 8 and 10 already contain eight “2”s within them as opposed to only two “5”s and there will be even more “2”s within the first 100 numbers.
= last digit of (1! + 2! + 3! + 4!) (because the rest have last digits zero and will not affect this result)
= last digit of (1 + 2 + 6 + 24)
= last digit of (33)
= 3
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As such, I found that there are 503 “5”s in the list altogether, versus much more than 1011 “2”s in the list. Since the number of “5”s is very limited, there can only be a series of 503 consecutive zeroes at the end of the list.
Keys to my working are:
1. For every 5 incremental of factorial, you get additional zero because the whole sum is multiply by 10 or having 5 multiply by any past even number.
2. Then we need to consider when the number reach in the hundreds as at 100,200,300, etc, instead of multiply by 10, the whole number is multiply by 100. So there is additional zero introduced as compared to point no1
3. Same thing with thousands, instead of multiplying by 100, we multiply by 1000. Again there is introduction of additional zero.
Say 24 * 25 = 600. Two zeroes are instantly introduced provided there are sufficient “2”s.