Twenty samples of subgroup size of 5 (n = 4) were collected for a variable measurement

Twenty samples of subgroup size of 5 (n = 4) 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