4 The weights in grams of a number of stones, measured correct to the nearest gram, are represented in the following table.
| Weight (grams) | \(1 - 10\) | \(11 - 20\) | \(21 - 25\) | \(26 - 30\) | \(31 - 50\) | \(51 - 70\) |
| Frequency | \(2 x\) | \(4 x\) | \(3 x\) | \(5 x\) | \(4 x\) | \(x\) |
A histogram is drawn with a scale of 1 cm to 1 unit on the vertical axis, which represents frequency density. The \(1 - 10\) rectangle has height 3 cm .
- Calculate the value of \(x\) and the height of the 51-70 rectangle.
- Calculate an estimate of the mean weight of the stones.