The cost of mailing a first-class letter is $0.46 for the first ounce and $0.20 for each additional ounce or portion of an ounce. y is the total cost for mailing a letter that weighs x ounces. which type of function model would be appropriate for this situation?

If y is the total cost to mail a letter, that is what your equation should be set equal to. 
y =
Then, just to be able to send the letter, you have to pay .46 before you even weigh it.
y = 0.46 +
Finally, you can weigh your letter to determine how much more you'd have to pay (it wouldn't be fair if a 1 ounce letter and a 5 pound letter costed the same!). The problem gives you the variable for ounces, so you would do
price x ounces.

y = 0.46 + 0.20x
(I'm not sure what the name of the equation is, but it is in y = mx + b form. I believe it is "slope intercept"? That should be somewhere in your notes. Hope this helps :) )

Rounding decimals is very similar to rounding other numbers. if the hundredths and thousandths places of a decimal is forty-nine or less, they are dropped and the tenths place does not change. for example, rounding 0.843 to the nearest tenth would give 0.8. , your welcome !