Welcome and Thanks for using Just Answer.
The z- score such that 6% of the area under the normal curve lies above it is
z = 1.5548
μ = 18 and σ = 4
x = μ + z * σ
x = 18 + 1.5548 * 4 = 24.22
The lowest race time that makes a horse loss his eligibility to participate in a
second round is 24.22 minutes
I hope that helps. Let me know if you need any clarifications on my answers. I will be happy to provide them. Else, please give a rating for my answer.