with rowMeans
> rowMeans(f[-1])
## [1] 6.6 17.4 10.8
You can also add another column to of means to the data set
> f$AvgSales <- rowMeans(f[-1])
> f
## name day1sales day1sales.1 day1sales.2 day1sales.3 day1sales.4 AvgSales
## 1 apple 2 2 2 22 5 6.6
## 2 orange 5 8 15 51 8 17.4
## 3 banana 4 6 24 13 7 10.8