15 cities, each of varying sizes, decide to have carbon-neutral public transport systems.
When the cities are arranged in size order from smallest to largest, the number of new electric buses they need follows an arithmetic sequence with first term (representing the smallest amount of buses needed for the smallest city) a buses and common difference d buses.
The 7ᵗʰ city in this progression needs 580 new electric buses and the largest city needs 1020 new electric buses.
Find the value of a and the value of d.
[5 marks]
