Must be a high proportion of smart and fast people on Letsrun.
A 1500 SAT is about in the 98th percentile. And if you're counting high school 5K PRs, well only a handful a year actually do that. Very few chances to do that other than XC, and seriously how many high school sub 15 XC times are there a year? How many on the track? Roads?
Maybe 10 to 20 high school kids actually do run under 15 in a given year (although those capable probably into the 100s or more, if you consider that a 9:20 3200 is close to equivalent for 15 flat for 5K).
Of those, what percentage could score 1500 on SATs. Even with a higher proportion of smart students who are runners (doubtful if you read Letsrun for more than 5 minutes), you might not get more than 10% or 15% of the top runners scoring that high (and that's being very optimistic).
Likewise, how many 15:30 high school runners are going to score 1530 (or 1550 if you're going by numbers) on SATs?
A pretty rare double, no doubt it happens but probaby not more than a few dozen student-athletes a year.
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High school:
SAT - 1050 (old school)
5K - 19:40
Post grad:
GRE - 1290 (1960 with analytical)
5K - 15:15