Determine if the following "proof" is valid or not. If it is valid, then give a justification for each step. If it is not, identify the error and explain why it is incorrect. Conjecture: For all propositions p, q, and r, p - -(q - r) = (p - q) A (p- -r). Proof: p - -(q - r) =p - -(-q vr) Ep - 79 A -r Ep- q A -r Empvan ar E (-P V q) A ( - P V ar) = (p - q) A ( p - -r) Question 10. Determine if the following "proof" is valid or not. If it is valid, then give a justification for each step. If it is not, identify the error and explain why it is incorrect. Conjecture: All multiples of 15 are multiples of 3 and multiples of 5. Proof: Let x be a multiple of 15. Then x = 15k for some k E Z. Then x = 5(3k) = 3(5k). Therefore, x is a multiple of 5 and a multiple of 3