The key to a is to first convert the percentages to ratio. Thereafter, find the common ratio item which is prawns. Multiply one or both of the ratios such that the common item is represented by the same number of units, then you will be able to find the ratio required by the question.
The second part requires understanding of what you are finding (eg 10 units is 1kg cost of prawns so you have to multiply by a certain number to find the total cost for that particular mass of item)