Buses leave halifax to elland every 9 minutes. buses leave halifaxto sowerby bridge every 12 minutes. a bus and a bus to sowerby bridge both leave halifax at 9am. at what time will a bus to elland and a bus to sowerby bridge next leave halifax at the same time.

hello from mrbilldoesmath!

answer:

a(n) - a(n-1) -3

discussion:

this ones a bit interesting. given a(n) = 5 - 3n consider this:

a(n-1) = 5 - 3(n-1)

          = (5 - 3n) + 3             => but a(n) = 5 - 3n, the first term

          = a(n)   + 3                 => subtract 3 from both sides

a(n-1) -3 = a(n)  

or

a(n) - a(n-1) -3

you,

mrb