[R] how to find the mean and sd :(

[massmatics]

I need help on 4b please :( 
4.) ‘Warpbreaks’ is a built-in dataset in R. Load it using the function
data(warpbreaks). It consists of the number of warp breaks per loom, where a
loom corresponds to a fixed length of yarn. It has three variables namely,
breaks, wool, and tension.

a.)	Write a code (Hint: a logical expression) that extracts the observations
with wool type A and tension level M. Assign it to the object AM.warpbreaks
*(This is done)*

b.) For the ‘AM.warpbreaks’ dataset, compute for the mean and the standard
deviation of the breaks variable for those observations with breaks value
not exceeding 30.

data(warpbreaks)

warpbreaks <- data.frame(warpbreaks)

AM.warpbreaks <- subset(warpbreaks, wool=="A" & tension=="M")

*> mean(AM.warpbreaks<=30)

>sd(AM.warpbreaks<=30)*

This is what I understood this problem and typed the code as in the last two
lines. However, I wasn't able to run the last two lines while the first 3
lines ran successfully. Can anybody tell me what is the error here? Thanks!
:)



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