An express train and a local train start out from the same point and the same time and travel in opposite directions. the express train travels twice as fast as the local train. after 4 hours they are 480 km apart. what is the average speed of the express train?

An express train and a local train start out from the same point and the same time and travel in opposite directions. the express train travels twice as fast as the local train. after 4 hours they are 480 km apart.

3x2y + 3xy (cannot be added)

3x2y + (–12x2y) = -9x2y  

3x2y + 2x2y2 (cannot be added)

3x2y + 7xy2 (cannot be added)

3x2y + (–10x2) (cannot be added)

3x2y + 4x2y = 7x2y

3x2y + 3x3 (cannot be added)

Step-by-step explanation:

Adding and subtracting a monomial requires having the same variables. No matter how big or small their coefficient is, if their variables do not match, they cannot be added or subtracted. The crucial part in adding or subtracting monomials is their sign. If the signs are the same, retain the sign. If the signs are different, subtract and keep the sign of the larger number.