Savings income：（18000*100/80)=22500, this is molly's actual bank interest income.

then Less: PA(personal allowance): 8105

then you get: taxable income: (22500-8105) 14395

20%income tax rate applies: so income tax payable will be (14395*20%) 2879.

However we also noticed that an income tax has been deducted from the income we shall received. an income tax already paid is 22500*20%=4500.

Then we can conclue there is an income tax receivable for Molly: 4500-2879=1621

So 1621 is the answer。

P.S: 顺带那中文给楼主备注一下，received bank interest是税后收入，所以要先计算税前收入，在计算income tax payable。因为扣得税比应交的税要多，所以产生了income tax receivable。