A test on aggregation of log-returns

Data scientists often use the aggregations that you have learned so far in combination with summary statistics to extract even more insights from data. Functions that calculate summary statistics include mean(), median(), and var().

The object sp contains daily log-returns for the S&P 500 index for the period 1960-2015; it is loaded in your workspace. To three decimal places, what is the average quarterly log-return for the S&P 500 from 1990-2010?