Cumilitive frequency just mean 'adding it up as you go along'
Draw a table with 3 rows and 6 columns. Row one will be the weekly profit, Row 2 will be the frequency. Row 3 will be the cumilative frequency. Add up the frequencies as you go. (I.e. 6, 12, 24, 35, 45, 50)
You then want to plot a graph with cumilative frequency up the y axis and weekly profit along the x-axis (the point you plot will be the cumilitive frequency and upper limit of the weekly profits - i.e. (6, 10);(12, 20);(24, 30) etc join the points up with a curve (the curve should look like a very gentle 's' shape)
Median is the middle value, exactly half way up the y-axis and then read across to your curve, read down to find the x-axis value. 80th percentile requires you to split your cumilative frequency data into 100 groups ..fairly easy as your total is 50, (80/100 is the same as 40/50) so simply read across from 40 on your y-axis to the curve then read down to find the x-axis value.
Hope this helps even without the graph, if I have the time I'll draw the graph for you and post a picture later. Good luck.