answer: 43 grams.
This problem is a good example to understand the law of conservation of mass. This law states that mass can neither be created nor destroyed , meaning that the final mass is the same that what you started with..
You didn't measure the weight of the oxygen that entered the reaction, so the weight of the rustic nail, plus the weight of the oxygen, will be the equal to the final mass.
100 +43 = 143g