Answer
Briscoe-- 216.2 mph
Castroneves--216.9 mph
Work Step by Step
Helio: 2.5 miles, .694 mph faster than Briscoe
Briscoe: 2.492 miles
distance = rate * time
Helio: $2.5 = (r+.694)*t$
Briscoe: $2.492 = r*t$
$2.5 = (r+.694)*t$
$2.5/(r+.694) = (r+.694)*t/(r+.694)$
$2.5/(r+.694) = t$
$2.492 = r*t$
$2.492/r = r*t/r$
$2.492/r = t$
$2.5/(r+.694) =2.492/r$
$2.492 (r+.694) = 2.5r$
$2.492r + 1.729448 = 2.5r$
$1.729448 = .008r$
$1.729448/.008 = .008r/.008$
$r = 216.181$
$r+.694$
$216.181+.694$
$216.875$