Final answer:
It's more practical to measure an object with mass of 15,000 kilograms in tonnes rather than grams for simplicity and ease of understanding. The mass would be 15 tonnes, whereas in grams it would unwieldy be 15,000,000 grams.
Step-by-step explanation:
When determining which unit is more sensible to measure a given mass, selecting a unit that results in a small and manageable number is practical. In the case of an object with a mass of 15,000 kilograms, expressing this mass in tonnes would be more sensible than in grams. One tonne is equal to 1,000 kilograms, making the mass of the object 15 tonnes, a much more manageable number than 15,000,000 grams.
The standard unit of mass in the SI system is the kilogram (kg). However, for very large masses like the object in question, using tonnes simplifies the number without changing the scale of measurement significantly. In contrast, using more granular units like grams can result in unnecessarily large and unwieldy numbers, which could complicate calculations and communication.