The following table gives the average and range in kilograms for tensile tests on an improved plastic cord. The subgroup size is 4. Determine the trial central line and control limits. If any points are out of control, assume assignable causes and calculate revised limits and central line. Subgroup Number X R Subgroup Number X R 1 476 32 14 482 22 2 466 24 15 506 23 3 484 32 16 496 23 4 466 26 17 478 25 5 470 24 18 484 24 6 494 24 19 506 23 7 486 28 20 476 25 8 496 23 21 485 29 9 488 24 22 490 25 10 482 26 23 463 22 11 498 25 24 469 27 12 464 24 25 474 22 13 484 24.