if 10% = 4.60 then by adding 4.60 to 4.60 he doubled the percent 10% +10% = 20%
he should have taken half of the 4.60 for the 5% since 5 is half of 10
half of 4.60 = 2.30
then add them 4.60 +2.30 = 6.90 = 15%
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