Determine the last two digits of the product of the squares of all positive odd integers less than .
Solutions — 2
Solution 1
Since the product of the odd integers less than contains as a factor, it is clearly divisible by . Also, since it is odd, its last two digits have to be or . So the product is either of the form or of the form . In either case, the last two digits of the squared product (which is the same as the product of the squares) are :
or
In fact, we see that the last three digits have to be .
Solution 2
Our number has as a factor, so its last two digits are , , or . Moreover, it is the square of an odd number, which we can write as . This means that the remainder upon division by has to be . Now note that , , and leave a remainder of , , , respectively when divided by . This means that the last two digits are in fact .
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