First cross-multiply to give both fractions the same denominator, then add the fractions: 1/3 + 1/4 = 4/12 + 3/12
Then add these fractions to find that she has 4/12 + 3/12 = 7 /12 of a bottle of shampoo left.
Then divide the total amount of shampoo she has by the amount she uses per day to find the number of days the shampoo will last: 7/12 / 1/12/day = 7 days
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