Probability that the first candidate solve the question correctly, `P(A)= 5/7`
Probability that the second candidate solve the question correctly, `P(B)= 4/7`
Probability that the third candidate solve the question correctly, `P(C)= 3/7`
Probability that at least two candidates solve correctly, `P(E) = P(A)P(B)P(barC)+P(A)P(barB)P(C)+P(barA)P(B)P(C)+P(A)P(B)P(C)`
`=>P(E) = 5/7**4/7**4/7+5/7**3/7**3/7+2/7**4/7**3/7+5/7**4/7**3/7`
`=>P(E) = 80/343+45/343+24/343+60/343 = 209/343`
So, required probability is `209/343`.