The cost of 1 purse and 1 watch is £27. The cost of 2 purses and 1 watch is £41.
a) How much does 1 purse cost?
b) How much does 1 watch cost?

1 purse and 1 watch is 27€
2 purse and 1 watch is 41€

So we can see the watch is constant means 1 watch is every time but purse is increase by 1 in the next expression

So 41-27 = 14€ is one purse
because the watch is every time one, so the next value increase is by purse

1purse and 1watch = 27
We know,
1 purse is 14€
So 27-14=13€ is one watch

We can check by
1 purse is 14 so two purse is 14*2=28
28+13=41
Which is same as they said in second expression

Hope it help you!¡