Paul travels from Rye to Eston at an average speed of 90 km/h. He travels for T hours. Mary makes the same journey at an average speed of 70 km/h. She travels for 1 hour longer than Paul. Work out the value of T

As distance is equal to speed multiplied by time, the distance Paul has traveled is 90 multiplied by T, or 90T.
As Mary is making the same journey, she travels the same distance. The distance she travels is obtained the same way, but she took 1 hour longer and travelled slower. So the calculation is 70 multiplied by (T+1), or 70(T+1), or 70T+70.
As they both travelled the same distance, the two distances must be equal, so 70T+70=90T.
If we take away 70T from both sides of the equation we are left with 70=20T.
To get the value of T we just divide by 20 to get T=3.5.