Question 132924
Let x=amount of pure thyme that needs to be added

Now we know that the amount of pure thyme in the 12 g mixture (0.25*12) plus the amount of pure thyme that is added (x) has to equal the amount of pure thyme in the final mixture (0.40(12+x)). So our equation to solve is:

0.25*12+x=0.40(x+12)  get rid of parens and simplify

3+x=0.40x+4.8 subtract 3 and also 0.40x from each side

3-3+x-0.40x=0.40x-0.40x+4.8-3 collect like terms
0.60x=1.8  divide both sides by 0.60

x=3 gms--------------------amount of pure thyme needed

CK
0.25*12+3=0.40(3+12)

3+3=0.40(15)
6=6

Hope this helps---ptaylor