Any number.

Final price with Increase is 140 + 28 = $ 168.

Her mouth

A river.

Stop.
All the words are anagrams of each other.

Surprisingly, there are several nine-letter words that can have one letter removed in each round to make a new word all the way to one letter, which must be ‘a’ or ‘i’. Startling is the most common answer, but I’ve included the other words I’m aware of. I don’t include plural words, like cleansers, drownings, splatters, starvings, trappings and wrappings because it’s kind of cheating.
startling
Remove the l to make starting (or remove the t to make starling)
Remove the t to make staring
Remove the a to make string
Remove the r to make sting
Remove the t to make sing
Remove the g to make sin
Remove the s to make in
Remove the n to make I

splitting
Remove the l to make spitting
Remove the p to make sitting
Remove a t to make siting
Remove the first i to make sting
Remove the s to make ting
Remove the g to make tin
Remove the t to make in
Remove the n to make I

stringier
Remove the r to make stingier
Remove the i to make stinger
Remove the t to make singer
Remove the r to make singe
Remove the g to make sine
Remove the e to make sin
Remove the s to make in
Remove the n to make I

strapping
Remove the s to make trapping
Remove the t to make rapping
Remove the p to make raping
Remove the r to make aping
Remove the a to make ping
Remove the g to make pin
Remove the p to make in
Remove the n to make I

This one has no solution. Unless we are complicating it with relativity theory - time and space. But to keep it simple, you can't reach the desired average speed under the given circumstances.

Skull.

Correct, the word 'what' has 4 letters in it, 'yet' has three, 'sometimes' has 9, 'then' has 4, 'rarely' has 6, and 'never' has 5.

Darkness

The chef. Mr Brown was killed in the afternoon and yet the chef claimed he was making breakfast?

The word "And"