TASK DEFINITION: In this task, find the most appropriate number to replace the blank (indicated with _ ) and express it in words.
PROBLEM: Optical isomers differ in the arrangement of _ groups around a chiral carbon.

SOLUTION: four

PROBLEM: Tobacco kills four million people a year, one death every _ seconds.

SOLUTION: eight

PROBLEM: Most people require at least seven to _ hours of sleep per night.

SOLUTION:
eight