How to make 25/3 in simplest form

Which of the following is most likely the next step in the series? a3z, b6y, c9x, d12w, е15v, f18u

In which form is the following function written? y = - 2(x – 3)(x + 5)

The distribution of the amount of money spent by students for textbooks in a semester is approximately normal in shape with a mean of $235 and a standard deviation of $20. according to the standard deviation rule, how much did almost all (99.7%) of the students spend on textbooks in a semester?