A 3-digit integer is called a 'V-number' if the digits go 'high-low-high' – that is, if the tens digit is smaller than both the hundreds digit and the units (or 'ones') digit. How many 3-digit 'V-numbers' are there?
This one wants a proof. Work it on paper, then check yourself against the publisher's own solution, linked below. Be honest about it: the record is only any use to you if it is.