The question you opened
What that wedding cost
A wedding seats 120 guests at 85 dollars a plate, and food is about a third of the budget.
About what does the whole wedding cost, in dollars?
about $30,600
Food is 120 times 85, which is 10,200. If that is a third of the budget, the whole thing is three times as much, about 30,600.
Two steps, both roundable. One hundred and twenty plates at 85 dollars is 10,200, which you can reach as 120 times 80 plus 120 times 5, so 9,600 plus 600. If food is a third of the budget, multiply by three to get about 30,600. Estimates are graded by range, and this one is comfortable.
Rounding 85 to 80 gives 9,600 and a total near 28,800. Rounding to 90 gives 10,800 and a total near 32,400. Both sit inside 20 percent of the answer, which is the standard tolerance for a question that begins with the word about. Precision beyond that is wasted, because the input itself, the phrase about a third, is not precise.
The structural move is called anchoring on a known part. You rarely know a total directly, and you often know one component and roughly what share it represents. Rent is often about a third of a household budget. Payroll is often half to two thirds of the running cost of a service business. Materials might be a fifth of a construction quote. Multiply the part by the reciprocal of its share and you have the whole.
The danger in the method is the share itself. If food is really a quarter rather than a third, the total is 40,800, and the estimate is off by a third. So the sensible output is a range rather than a point, and the honest way to say it is somewhere between 30,000 and 40,000. The transferable habit is to notice which number in an estimate carries the most uncertainty, and to spend your effort on that one. Here it is the share, and the plate price barely matters.
Technique: Price one part, then scale it