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Macedonian Mathematical Competitions

Number theory Difficulty 5.1 AIME, harder Prove it North Macedonia

How many numbers divisible by 30200830^{2008} are not divisible by 20200720^{2007}?

Solution

Since 302008=22008⋅32008⋅5200830^{2008} = 2^{2008} \cdot 3^{2008} \cdot 5^{2008} and 202007=24014⋅5200720^{2007} = 2^{4014} \cdot 5^{2007}, all the numbers divisible by 30200830^{2008} and not divisible by 20200720^{2007} are:

1) 2k⋅3l⋅5m2^{k} \cdot 3^{l} \cdot 5^{m}, l=1,2,…,2008l = 1, 2, \ldots, 2008, k,m=0,1,2,…,2008k, m = 0, 1, 2, \ldots, 2008 or 2008⋅200922008 \cdot 2009^{2} numbers,

2) 2k⋅5m2^{k} \cdot 5^{m}, m=2008m = 2008, k=0,1,2,…,2008k = 0, 1, 2, \ldots, 2008 or 1⋅2009=20091 \cdot 2009 = 2009 numbers.

Finally, there are 2008⋅20092+20092008 \cdot 2009^{2} + 2009 numbers divisible by 30200830^{2008} and not divisible by 20200720^{2007}.

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