Rounding to 50 or 500

It is tempting to subtract the result of (a) and (b) (54-45 = 9) to get (c), but the count includes 45 through 54. Subtracting would leave 45 out and be one short. Some students will approach problems (a) through (c) in an exploratory way, listing all the numbers 'close to' 50 and counting. Using this approach their most likely mistake is to leave off 50 itself; they may also include both 45 and 55 even when they are supposed to be adopting a consistent 'round up when the last digit is 5' rule.