One morning . the shirt shop sold 15 t-shirt. of the t-shirts sold, 1/5 were gray. the rest were white. how many white t-shirts were sold?

If 15 t-shirts were sold and 1/5 were grey,
     
                    then  4/5 of the shirts were white 

Now,   4/5 of 15 = 12
   
                ∴ of the shirts sold, 12 were white

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