Aplayground is being designed where children can interact with their friends in certain
combinations.
if there is 1 child, there can be o interactions.
if there are 2 children, there can be 1 interactions.
if there are 3 children, there can be 4 interactions.
if there are 4 children, there can be 9 interactions.
how many interactions will there be for 6 children?

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