(a) Bob has total income of £ 21,445 in 2017-18. None of his income is derived from savings or dividends and he claims the basic personal allowance. During the year he makes a payment which is deductible from total income of £200 (paid gross).

(b) Cathy's circumstances are precisely the same as Bob's except that her payment is required by law to be made net of basic rate tax, so she pays a net amount of £160.
Compute Bob and Cathy's income tax liabilities for 2017-18. Does it seem to make any difference whether a deductible payment is made gross or net?