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Question:

Let * be a binary operation of N given by ab = LCM(a,b) for all a,b ∈ N. Find 57.

Solution:

ab = LCM(a,b) for all a,b ∈ N
Here, a = 5, b = 7
Thus, 57 = LCM(5,7) = 5 × 7 = 35 (Since the two numbers are co-prime, their LCM becomes their product)