How can you check the correctness of a division answer that has no remainder

The triangles are similar. what is the value of x? show your work.

The quotient of 15 and a number is 1 over 3 written as an equation

Luigi is the first person to finish a 5k race and is standing 13 feet beyond the finish line. another runner, mario, is still running the race and has approximately 24 feet before he reaches the finish line. what is the distance between mario and luigi? explain through words, numbers or pictures how you found the answer.