The Chandlers move at a constant speed, so they accomplish the formula: V = D / T , where V is speed, D is distance and T is time, then they meet when distance covered by both of them will be the same, taking in account that Mrs. Chandler leaves 3.5 hours later but travels fester. Mr. Chandler speed is : Vmr=d/t=45 => d=45*t Mrs.Chandler speed is Vmrs=d/(t-3.5)=70 Solving the second equation: 70=45*t/(t-3.5) => 70*t-245=45*t => 70*t-45*t=245 t=245/25 = 9.8 hours We can check the solution by calculating the distance drived after that time: Mr. Chandler: d=45*9.8=441 miles Mrs. Chandler: d=70*(9.8-35)=441 miles Same distance, so then correct.