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    What amount of coal tar do you need to add to 250g of 10% coar ointment to convert it to 20% coal tar ointment?

    The answer is 31.25g.

    Could someone please explain how to do this, thanks xxx
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    Let x be the amount of tar to add. The total amount of new tar will be
    0.1*250 +x.
    Now, this needs to be 20% of the amount of tar in the new ointment, which we will have 250+x grams of. So we should have (250+x)*0.2 grams of tar in the new ointment, which has to be the same as the above, so we have
    (0.1*250 +x)= (250+x)*0.2
    rearranging gives
    25+x = 50+0.2*x
    rearranging again gives
    0.8x = 25
    so
    x=31.25.

    That's how I'd do it, but I'm a uni maths student, so I'm not sure how your teacher might have shown you how to do this.
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    sorry the formatting went funny
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    (Original post by girlygirl123234)
    What amount of coal tar do you need to add to 250g of 10% coar ointment to convert it to 20% coal tar ointment?

    The answer is 31.25g.

    Could someone please explain how to do this, thanks xxx
    The answer is x
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