A group of people went to a coffee shop. Each person chose a drink and a treat.

number of people who chose coffee: number of people who chose tea = 1:4

Of those who chose coffee,
number of people who chose a muffin number of people who chose a tart = 2:7

Of those who chose tea,
number of people who chose a muffin : number of people who chose a tart = 5:1

What fraction of the people chose a tart?