The 50/50 principle means that what you left from your monthly income after all your monthly expense. The left over money, only 50% or less of it should be use to repay loan. The remaining 50% or more need to be save up for saving or investment. For Exampe, if your monthly income is $50,000 HKD and monthly expense is $40,000 HKD. You will have $10,000 left every month, the maximum amount of money you can use to repay loan is $5,000 HKD and you should save up the remaining $5,000 HKD.