effective marginal rate of tax

Hello

Please can someone help with understanding how to calculate effective marginal rate of tax saved in this question . ( any question in general really, this one is for an example).

I dont understand how the effective rates on the first £500 and 4300 has been calculated as 20% and 60% .

Samson and Delilah are a married couple. They are both employed by Rope plc, and Delilah is also a partner in a
partnership. The following information is available in respect of the tax year 2019/20:
Samson
During the tax year 2019/20, Samson was paid a gross annual salary of £112,000 in respect of his employment
with Rope plc.
Delilah
(1) During the tax year 2019/20, Delilah was paid a gross annual salary of £184,000 in respect of her
employment with Rope plc.
(2) Throughout the tax year 2019/20, Rope plc provided Delilah with a petrol powered motor car which has a list
price of £67,200, and an official CO2 emission rate of 152 grams per kilometre. Rope plc does not provide
Delilah with any fuel for private journeys. Delilah was unable to drive her motor car for a period during the
tax year 2019/20 because of a skiing accident, and during this period Rope plc provided her with a chauffeur
at a total cost of £9,400.
(3) Rope plc provided all its employees with a hamper of groceries costing £42 each in December 2019.
(4) Delilah spent £70 in July 2019 on travelling by train to visit a customer of Rope plc. This amount was
reimbursed by Rope plc in August 2019.
(5) During the tax year 2019/20, Delilah donated £250 (gross) per month to charity under the payroll deduction
scheme operated by Rope plc.
(6) Delilah has been in partnership with Esther and Felix for a number of years. The partnership’s tax adjusted
trading profit for the year ended 31 December 2019 was £93,600. Esther is paid an annual salary of £8,000,
with the balance of profits being shared 40% to Delilah, 30% to Esther and 30% to Felix.
(7) During the tax year 2019/20, Delilah paid interest of £6,200 (gross) on a personal loan taken out to purchase
her share in the partnership.
(8) During the tax year 2019/20, Delilah made charitable gift aid donations totalling £4,864 (net).
Joint income – Building society deposit account
Samson and Delilah have savings in a building society deposit account which is in their joint names. During the tax
year 2019/20, they received building society interest totalling £9,600 from this joint account.
Required
(a) Calculate Samson and Delilah’s respective income tax liabilities for the tax year 2019/20.
Note. The following mark allocation is provided as guidance for this requirement:
Samson (4 marks)
Delilah (9 marks) (13 marks)
(b) Calculate Samson’s income tax saving for the tax year 2019/20 if the building society deposit account had
been in Delilah’s sole name instead of in joint names for the entire year.

Many thanks for your help