10.5% a year is 10.5/52=0.2019% a week. The compound factor is therefore 1.002019. We have to solve 25000*1.002019^x=30835.42, where x is the number of weeks. So, 1.002019^x=30835.42/25000=1.2334. Take logs of each side: x=log1.2334/log1.002019=104 weeks=2 years.