Finding boat age from carbon-14 activity decay

[Tangeton]

Homework Statement

The carbon in living matter contains fixed proportion of the radioactive isotope carbon-14. The carbon-14 in 1.00g of living matter has an activity of 0.250Bq. The half life of carbon-14 is 5730 years.
(a) Calculate (i) The decay constant in s^-1 (ii) the number of carbon-14 atoms required to produce an activity of 0.250Bq.
(b) When a plant dies the proportion of carbon-14 decreases due to radioactive decay. A 1.00g sample of carbon from an ancient boat has an activity of 0.0625Bq. Determine the age of the boat.

Homework Equations

A = yN
A = A₀e^-yt
A₀ = yN₀
A = A₀e^-yt

The Attempt at a Solution

I've worked out (a) (i) to be 2.48x10^-12 and (ii) N = 1.01x10¹¹

I don't know how to work out (iii) because I don't know how to work out N₀ or A₀. I found using A = yN --> N = A/y = 2.52x10¹⁰

How to find either A₀ or N₀ so that I can use A = A₀e^-yt or N = N = N₀e^-yt?

 

[Simon Bridge]

You do it using proportions.

i.e.
What fraction of the sample was C14 when it was new?
What fraction is it now?