If I am wanting to save $35 each week for something specific this would equal the following;

Fortnightly saving of $70 ($35 a week x 2 weeks)
Monthly saving of $140 ($70 a fortnight x 2 weeks)
Yearly saving of $1,680 ($140 a month x 12 months)

Why when i calculate $35 a week x 52 weeks (1 year period) do i get the sum of $1,820?

What am i doing wrong to get two different yearly figures? One of $1,680 and one of $1,820?

I am stumped and a little perplexed why i cannot figure out something as simple as this!?!