The following mistakes could be explained observed from the plotted bar chart presented in the attachment :
1.) The numeric label of the y-axis which shows the number of text messages isn't well scaled, the numbering did not start from 0, hence this is a defect in the scaling of the bar chart.
2.) Looking closely, the plotted bars do not ALIGN well in terms of spacing as the interval between each bar isn't consistent.
3.) Both the x and y axis of the graph are t labeled thus failing to add a good and meaningful description to the plot especially for first time viewers.
First, add 20 and 40. that gives you 60. then, you divide- 420/60. that gives you 7. then multiply 7 by 20, giving you 140, and multiply 7 by 40, giving you 280. then subtract: 280-140
P=28in because there are two sides that equal 6in and two other sides that equal 8. You multiply 6 and 8 by 2 which is 6*2=12 and 8*2=16. And then you add 12 and 16