Achemist needs 90 milliliters of a 25% solution but has only 16% and 43% solutions available

Let f be amount of 43% and  s be amount of 16%...

s+f=90, f=90-s

(.16s+.43f)/90=.25  using f found above we get:

(.16s+.43(90-s))/90=.25

.16s+38.7-.43s=22.5

-.27s=-16.2

s=60, and since  f=90-s, f=30

So the chemist will need to mix 60ml of 16% solution with 30ml of 43% solution to create 90ml of a 25% solution.