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.
