Alan

Combine the 85 and -43 into 42

?*12 + 42 = 246

Subtract 42 from both sides

?*12 = 204

Divide both sides by 12

? = 204/12 = 17

Just enter, for example, 3 + 1/2 and press =

$${\mathtt{3}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}} = {\frac{{\mathtt{7}}}{{\mathtt{2}}}} = {\mathtt{3.5}}$$

$$\frac{2}{3}(27y+9)=\frac{2}{3}\times 27y+\frac{2}{3}\times9 =18y+6$$

3x + x = 4x;   14/7 = 2; 38/5 = 7 + 3/5, so we have:

4x + 2 + 7 + 3/5 = 14

4x + 9 + 3/5 = 14

4x = 14 - 9 - 3/5

4x = 4 + 2/5

x = 1 + 1/10

$$x=1\frac{1}{10}$$

x = 11/10

x = 1.1

(Take your pick!)

No harm in having more than one reply ND!

What do you mean by "key factor"?

(5 is a prime number and has no factors other than itself and 1).

Multiplying through to get rid of the brackets this becomes:

2x - 60 + x + 20 + 10 = 6x - 210

or:  3x - 30 = 6x - 210

Add 210 to both sides:

3x + 180 = 6x

Subtract 3x from both sides:

180 = 3x

Divide both sides by 3:

x = 60

How about x = {11, 12, 13, 14, 15, 16, 17, 18, 19}?

Mean

Add up all the numbers and divide by how many there are.

Variance

First find the difference between each number and the mean.  Square each of these differences.  Add up all these squared differences. Divide the result by one less than how many there are (i.e. if you have 8 numbers, then divide by 7).

Standard deviation

This is just the square root of the variance.

Calculator

There is one on the home page of this site.

Let's see you have a go at these and then perhaps someone will check your answers.