The decay of a radioactive substance is modelled using the formula N=800e−ktN = 800e^{-kt}N=800e−kt Where N N\,N is the number of atoms after t t\,t years and k k\,k is a positive constant.
Write down the number of atoms when the substance started to decay.
Given it takes 12.5 years for half of the substance to decay, find the value of k k\,k to three significant figures.
Calculate the number of atoms left when t=25t=25t=25.
277 exam-style questions on Edexcel A Level Maths Exponentials and Logarithms, covering 14.1 Exponential Functions, 14.2 y = e^x, 14.3 Exponential Modelling, 14.4 Logarithms, 14.5 Laws of Logarithms, 14.6 Solving Equations using Logarithms, 14.7 Working with Natural Logarithms, and 14.8 Logarithms and Non-Linear Data. Each one has a worked solution and a mark scheme showing where the marks go.