Question

What is the cost of 400g at £1.50 per 1/2kg

Answer 1

Well 0.5kg is an awkward number, so let's double £1.50 and 0.5kg to get £3 for 1kg, or 1000g. 

So now, we can set up a proportion. (x is the cost) 

$\frac{3}{1000} = \frac{x}{400}$.

Now we multiply 400 on both sides to get 'x' on its own.

$\frac{3}{1000}$ × $400 = 1.2$

So now, we know the cost for 400g = £1.20.