15 minutes is on quarter of an hour, so if Nat rides at 10k/h, he will have ridden 2.5 km at the time Alex leaves. Alex rides 2k/h faster than Nat, so he will make up the 2.5 hr different in 1.25 hours, or 1 hour and 15 minutes. since Alex left at 10:15, they will meet at 11:30.