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\text { 5. From } 1,2,3,4,5,6,7,8,9,10
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choose any two numbers, find the probability that their product is a multiple of 5.
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(Answer) $\mathrm{P}=\frac{\mathrm{C}_{8}^{1} \cdot \mathrm{C}_{2}^{1}+\mathrm{C}_{2}^{2}}{\mathrm{C}_{10}^{2}}=\frac{17}{45}$ )