Grouping by interval

Hello,

I have a dataframe of say 20 lines with one line per individual. I want to group these 20 individuals
by length class (eg. of 5cm) and get the mean value of all the other variables (eg VarA and VarB) for each length class

My dataframe is as follow:

Length <- 10:30
VarA <- seq(1000,1200,10)
VarB <- seq(500,700,10)
Data <- cbind(Length,VarA,VarB)


And I want to get something like:


Length Class      Mean VarA       Mean VarB
[10-15[               1020                520
[15-20[               1070                570
[20-25[               1120                620
[25-30]               1175                675


Would you have any suggestions how to do that ?
Many thanks.


Sonia.

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