A usually requires calculations that are too complicated to do by
hand.

computer model

Explanation:

Computer models are cheaper to set up than alternative methods that could be used to predict what will happen in a system, ex. building a prototype. Other benefits include being able to: make alterations and quickly see the outcomes.

I'm answering this, so I can message you and help with with art.

it means there are 3 atoms of oxygen

i believe the answer is three water molecules