Math Help

Twenty samples of subgroup size 3 (n = 2) were collected for a variable measurement. Determine the upper control limit (UCL) for an R-chart if the mean of the sample ranges equals 6.4.
 
Factors for calculating control limits
n             A2           D3           D4
2              1.880     0              3.267
3              1.023     0              2.574
4              0.729     0              2.282
5              0.577     0              2.114
6              0.483     0              2.004