Answer: a) $620 ; b) 7.75 %
Step-by-step explanation: To calculate the eran money y due to the investment we have to add the two quantities.
the 4% for $2000 and 9% for $6000
This is: $80+$540= $620
Finally to determine the percent interest for his total investment, we have to divide the erned money over the total money, that is:
% earned=$620/$8000= 0.0775 the 7.75 %