When do you carry the high number in vertical multiplication

When i add money am i supposed to make it like this 7+12 or 7.00+12.00 because i got 19 dollars for my answer

What values of x satisfy the inequality x^2-9x< 0? how is your answer shown on the graph?

Is the product of two rational numbers irrational or rational? first, make a hypothesis by multiplying two rational numbers. then, use variables such as x=a/b and y=c/d and the closure property of integers to prove your hypothesis.