Let's do the work shall we....
this is going to be a ratio problem.
Striped Marlin:
70 miles/ One hour (Or 60 minutes)
Sailfish:
40 miles/ 30 minutes.
Since 30 minutes is only one half of 60 minutes, all we have to do is multiply the sailfish ratio by 2 in order to receive the correct mph.
(40miles/30 minutes) x2 = (80 miles/60 minutes(or one hour)).
Therefore, the sailfish is faster! 80 miles per hour is faster than the 70 mph marlin.