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)
[tex] \frac{3}{1000} = \frac{x}{400} [/tex].
Now we multiply 400 on both sides to get 'x' on its own.
[tex] \frac{3}{1000} [/tex] × [tex]400 = 1.2[/tex]
So now, we know the cost for 400g = £1.20.
