So the beach is eroding at a rate of 4 centimetres per year...
Firstly, let's think about what a centimetre is...
A centimetre is actually equal to 10 millimetres. This would mean that 4 centimetres is equal to 40 millimetres.
Let's change the sentence we started off with to:
"A particular beach is eroding at a rate of 40 millimetres per year."
Now we know that there are 365 days in a year, so we can transform the sentence just above into:
"A particular beach is eroding at a rate of 40 millimetres per 365 days."
From here we can tackle the problem and come up with a solution...
Let's divide 40 by 365 to find out how much the beach is eroding every day.
40/365 is equal to 0.11 (to 2 decimal places)
This means that the beach is eroding at a rate of roughly 0.11 millimetres per day.