Can someone help me with this math problem

Acircle has a circumference of 28.36 units what is the diameter of the circle

Expresa commuter train leaves downtown sation and travels at a average speed of 55 miles per hour towards the north side sation, which is 50miles away. thirty minutes later, express commuter train #7 leaves north side and travels at a average speed of 35miles per hour towards downtown sation. at the moment the two trains pass each other, how far(in miles) is train #12 from the downtown sation and how long(in minutes) has the #12 train been traveling

The measures of the legs of a right triangle are 15m and 20m . what is the length of the hypotenuse