Use the lower-tail quantile to set the new filling mean

A manufacturer produces margarine packets with normally distributed mass, mean 502 g and standard deviation 0.8 g. Find the proportion containing less than 500 g.

The mean is increased to μg and the standard deviation is unchanged. Only 1 packet in 1000, on average, now contains less than 500 g. Find μ correct to 1 decimal place.