i just cant find a formula to do this

The weight of medium-size tomatoes selected at random from a bin at the local supermarket is a

random variable with mean ? = 10 oz. and standard deviation ? = 1 oz.

Suppose we pick four tomatoes from the bin at random and put them in a bag. Define

the random variable Y = the weight of the bag containing the four tomatoes. What is the

standard deviation of the random variable Y?