10 km / h = 10000m/ 60 min = (500/3) m / min
7 km / h = 7000m / 60 min = (350/3)m/ min
(500 + 350) / 3 = 850 / 3 = the number of combined meters they run every min
And 400 m = ( 1200 /3 ) meters
So....every minute they run ( 850 / 1200) = 17/24 of a lap combined
1 min = 17/24 lap
24/17 min = 1 lap
So they will pass each other every 24/17 min
So in 60 min they will pass each other
integer 60 / [ 24 /17] = integer [ 60 * 17 ] / 24 = 42 times
