Velocity(simple) problem. From point A a runner starts to run with constant velocity. Half an hour later, from the same point A, starts running a second person which his velocity is bigger in 2 (kph) than the first runner. The second runner achieved the first at 30 km from point A. Q: What is the velocity of the first runner?

It helps to write down what the variables stand for. It looks (from your comment) that $v$ is the speed of the first runner and $t$ is the time they meet, measured from the time the first started. Then your equation $(v+2)(t+1/2)=30$ is incorrect, as the second runner has been running for time $t-1/2$.