How does using a model me multiply decimals by whole numbers?

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

If a baby uses 15 diapers in 2 days, how many diapers will the baby use in a year?

The marriott family bought a new apartment three years ago for $65,000. the apartment is now worth $86,515. assuming a steady rate of growth, what was the yearly rate of appreciation? what is the percent of the yearly rate of appreciation?