You don't need to "figure out" how much you paid for chicken; it's printed on the receipt. The only thing to "figure out" is "what's the cost per pound if I paid $7 for a 2.3 pound chicken" and maybe you want to do that arithmetic, but for a lot of people, "this chicken cost $7 and I estimate that it will feed me and my partner for maybe 2-3 meals" is the only 'calculation' that needs to happen.

Larger calculations like "how much did I spend last month on groceries" are easily assessed simply by looking at a credit card statement. Being financially responsible does not necessarily entail knowing the exact dollar-to-calorie or dollar-to-weight ratio of every item in your shopping cart.