require(pixmap)
XXa<-attributes(read.pnm("D202-1_003a.ppm"))
XXb<-attributes(read.pnm("D202-1_003b.ppm"))
XXc<-attributes(read.pnm("D202-1_003c.ppm"))

XXred<- (XXa$red+XXb$red+XXc$red)/3
XXblu<- (XXa$blue+XXb$blue+XXc$blue)/3
XXgre<- (XXa$green+XXb$green+XXc$green)/3
dat<- array(c(XXred, XXgre, XXblu), c(attributes(XXred)$dim,3))
XX<- pixmapRGB(dat)

write.pnm(XX, file="D202-1_003avg.ppm")
cat("Done\n")