Final answer:
Greg is the shortest of them all.
Step-by-step explanation:
To determine who is the shortest among Greg, John, and Mark, we need to compare their heights. According to the information given, Greg is 3/4 as tall as John, and Mark is 1 1/4 as tall as John. To make a comparison, we can convert the fractions to decimals: 3/4 is equal to 0.75, and 1 1/4 is equal to 1.25. Since Greg is shorter than John and Mark is taller than John, Greg would be the shortest of them all.