To find the average in this case, you would divide the total earnings by the number of hours.
Since she mowed lawns for 3 hours, she earned $20.40 from that (3*6.8)
Since she washed windows for 2 hours, she earned $12.40 (6.2*2)
So if we add 20.4 and 12.4 we get $32.80
And if we divide 32.8 by 5 hours, we get $6.56 per hour
Hope this helps