I'm not entirely sure if this is the right way of doing it)
So you add the two numbers in the ratio (5 and 7) which makes 12 and you divide £60 by that number
60 divided by 12 is 5 so you times that by 5 to get Mary's value (£25) and by 7 to get Bob's (£35)
You can check this by adding the 2 numbers and if it adds up to £60, you're probably right