In order to find the mean, consider that there are an equal number of girls and boy, then, if n is the number of boys, n is also the number of girls.
Then, for the mean, you have:
![\operatorname{mean}=\bar{x}=(0.85n+0.91n)/(n+n)=(1.76n)/(2n)=(1.76)/(2)=0.88]()
Hence, the mean is 88%
To calculate the standar deviation, just consider that percentage for both boys and girls are 0.03 apart of the mean (0.88). Then, it is not necessary to calculate explicitly the standard deviation, because in this case, the 'distance' between the data and the mean is the same.
Hence, the standard deviation is 0.03, or simply 3%