Hi Sri,
My apologies for not answering both questions in the initial post.
For the second question, you can absolutely calculate the average of the three groups, but you can also "eyeball" the table to tell which group has the smallest average study time. It's important to note that each group consists of 11 students each.
Notice how most of the points are clustered around 2 hours and 3 hours.... Groups A (the white circles) and B (the blue squares) have many terms each that are greater than 3, which would push their averages "up"; Group C (the black circles) has only 1 term above 3 (just one 4) and 2 terms at the lowest level (1 hour), which means that this must be the group with the lowest average study time.
While certain chart/graph/table questions in the IR section will require a calculation, many will not. Keep an eye out for questions that require a specific numerical answer vs. those the require a non-numeric answer. The ones that don't require a numeric answer are ones that you can likely answer by spotting a pattern.
GMAT assassins aren't born, they're made,
Rich