The table shows the distribution of masks obtained by students in an examination. 

Marks	50 - 54	55 - 59	60 - 64	65 - 69	70 - 74	75 - 79	80 - 84	85 - 89
Frequency	5	15	20	28	12	9	7	4

Using an assumed mean of 67, calculate, correct to one decimal place. the

a) Mean

b) Standard deviation of the distribution