Mortgage interest calculation question

Millennium

[FONT=&quot]Hi all[/FONT]
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[FONT=&quot]What a fantastic forum this is! So much so that I have decided to start paying off my mortgage. I have a question for you maths gurus though.[/FONT]
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[FONT=&quot]Using the BBC mortgage calculator <http://www.bbc.co.uk/homes/property/mortgagecalculator.shtml>[/FONT]
[FONT=&quot]I get the following:[/FONT]
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[FONT=&quot]Sub-account A is at £36,000 at 5.99% Interest only.[/FONT]
[FONT=&quot]Sub-account B is also at £36,000 at 3.99% as a repayment with a remaining 15 year term.[/FONT]
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[FONT=&quot]Sub-account A[/FONT]
[FONT=&quot]As a repayment works out at £308.68 as a repayment and £179.70 as interest only.[/FONT]
[FONT=&quot]Sun-account B[/FONT]
[FONT=&quot]As a repayment works out at £269.63 as a repayment and £119.70 as interest only.[/FONT]
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[FONT=&quot]Obviously I want to hit the 5.99% portion first, but if I make account A repayment and account B interest only I get this:[/FONT]
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[FONT=&quot]The 5.99% part works out as £428.39 per month (£308.68+£119.70) being £20.95 less per month but hitting the higher interest rate.[/FONT]
[FONT=&quot]The 3.99% part works out as £449.33 per month (£179.70+ 269.63)[/FONT]
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[FONT=&quot]I find this odd! If this is correct I would be better to switch the 5.99% to a repayment (but continue to pay off this part in monthly lump sums plus the £20 saved!), and leave the 3.99% at interest only until I have paid off the 5.99%.[/FONT]
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[FONT=&quot]Am I doing something wrong?[/FONT]