Please answer without the use of excel manual solving.

Your uncle holds just one stock, East Coast Bank (ECB). You agree that this stock is relatively safe, but you want to demonstrate that his risk would be even lower if he were more diversified. You obtain the following returns data for West Coast Bank (WCB). Measured by the standard deviation of returns, by how much would your uncle's risk have been reduced if he had held a portfolio consisting of 70% in ECB and the remainder in WCB