Write an equation that you can use to find the ideal number of cups of water, w, that you should add when you have a container with s cups of sand

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

Math it's appreciated. (picture provided)

Erin bought christmas cards for $2 each to send to her family and friends. which variable is the dependent variable?