For Q2, the answer should be 15 years. The question is wrong. Rule of 72 should be '72/interest rate' for an investment to double. You can try it yourself on your calculator. Therefore, at 8%, it will take 72/8 = 9 years for first portion to double. Then 72/12 = 6 years to double again at 12%. Then 9+6 = 15 years.

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Q2 - is it 4+6 = 10 ?

Besides it doesn't make sense that at 8% interest, it takes only 4 years to double as opposed to 6 years when interest rates are at 12%.

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Best Regards,

Paul