Answer:
 2.5 miles
Step-by-step explanation:
The relation between time, speed, and distance is ...
 distance = speed × time
We can define t to be Stanley's swimming time. Then t+0.5 was his running time, and 2(t+0.5) was his biking time. His total distance covered is ...
 64 = 9(t +0.5) +16(2(t +0.5)) +2.5(t)
 64 = 43.5t +20.5 . . . . . . . simplify
 43.5 = 43.5t . . . . . . . . . subtract 20.5
 t = 1 . . . . . . . . . . . . . . divide by the coefficient of t
Stanley swam for 1 hour, so the distance he covered while swimming was ...
 (2.5 mi/h)(1 h) = 2.5 mi
Stanley covered 2.5 miles while swimming.
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Additional comment
Stanley ran for 1.5 hours, covering 9×1.5 = 13.5 miles. He biked for 3 hours, covering 16×3 = 48 miles. His total distance was 2.5 +13.5 +48 = 64 miles, as given.
