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How do you use the additive inverse to evaluate an expression that uses subtraction in a full sentence

Ageologist had two rocks on a scale that weighed 4.6 kilograms together. rock a was 0.5 of the total weight. how much did rock a weigh?

Arestaurant chef made 1 1/2 jars of pasta sauce. each serving of pasta requires 1/2 of a jar of sauce. how many servings of pasta will the chef be bale to prepare using the sauce?