When lines are perpendicular their slopes flip and?

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?

18 16 11 45 33 11 33 14 18 11 what is the mode for this data set

What is the point of inflection for the function f(x)=2cos4x on the interval (0, pi/2)