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

Could someone please explain how to do this, thanks xxx
2. 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.
3. sorry the formatting went funny
4. (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?

Could someone please explain how to do this, thanks xxx
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