The shark went from 20 miles per hour to a speed of 2*20=40 miles per hour.
The shark increased its speed by 40-20=<<40-20=20>>20 miles per hour.
Half of the shark's increase in speed is 20/2=<<20/2=10>>10 miles per hour.
Thus the pilot fish had increased its speed to 20+10=<<20+10=30>>30 miles per hour.
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