Final answer:
A beaker with 30% glucose solution will have 70% water since the two percentages must total 100%.
Step-by-step explanation:
If a beaker contains 30% glucose, it will contain 70% water. The reasoning behind this calculation is based on the fact that a solution's percentages must add up to 100%. Thus, if 30% of the solution is glucose, then the remaining percentage must be water, which is 100% - 30% = 70% water.