Numbers divisible by 3 = {3, 6, 9, ...... 300}
Numbers divisible by 7 = {7,14, ........ 294}
Numbers divisible by both 3 and 7 (i.e. multiples of 21) = {21, 42,.... 294}
Required probability = $$\frac{100 + 42 - 14}{300} = \frac{128}{300} = \frac{32}{75}$$
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