Answer:
57.885
Step-by-step explanation:
For such calculations a probability calculator is very helpful. The one in the attachment shows the 87th percentile to be 57.885.
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A table of the standard normal distribution will tell you the 87th percentile corresponds to a z-value of 1.12639. Then the X value is ...
X = Zσ +μ = 1.12639(7) +50 = 57.885
