At a hardware store, each wooden plank costs $3.57. how much do 12 planks cost?

The sum of two numbers is 12 and their difference is 18 what are the two numbers

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

What is the product? a-3/15a × 5/a-3