$$\frac{1}{3} - \frac{1}{12} = \frac{1}{x}$$

$$\frac{4}{12} - \frac{1}{12} = \frac{1}{x}$$

$$\frac{3}{12} = \frac{1}{x}$$ 

Now by cross multiplying:

$$3x = 12$$

$$x = 4$$

Make 1 parcel = X

4X = 124 kg

X = 124/4 kg

X = 31 kg

$$\frac{2}{7}\times\frac{1}{7} = \frac{2\times{1}}{7\times{7}} = \frac{2}{49}$$

This is already in its simplest form: -270k^2 + 405k

Although it can be factorised:

-270k^2 + 405k ---> -135k(2k - 3)

X*6 = 12

X = 12/6

X = 2

Or you could look at it like this:

You have 1 eighth,

Multiply this by 2 and what do you get? 2 eighths, or 2/8, which can be simplified down to 1/4, which equals 0.25.

225/324 = 25/36

(The highest common factor is 9)

-2/(1 1/2) = -2/(3/2) = (-2/1)*(2/3) = -4/3 = -1 1/3

Anything to a negative power is the same as 1 over that number to a positive power: 

$${{\mathtt{a}}}^{-{\mathtt{1}}} = {\frac{{\mathtt{1}}}{{{\mathtt{a}}}^{{\mathtt{1}}}}}$$

So for (1/4)^-1 . . . 

$$\frac{1}{(\frac{1}{4})^1} = \frac{1}{1}\times(\frac{4}{1})^1 = 4$$

Note that when you divide by a fraction you instead flip the denominator and numerator. When you get more practiced at this you can go straight from (1/4)^-1 to 4, as long as you know the process behind it.

In the same way . . .

$$\frac{1}{(\frac{1}{4})^2} = \frac{1}{1}\times(\frac{4}{1})^2 = \frac{4^2}{1^2} = 4^2 = 16$$

$$(\frac{1}{4})^2 = \frac{1^2}{4^2} = \frac{1}{16}$$