An isotope of cesium (cesium-137) has a half-life of 30 years. If 4.0 mg of cesium-137 decays over a period of 120 years, how many mg of cesium-137 would remain?

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The third stanza jumps way forward in time. Aengus is now an old man, and he's spent his whole life looking for the "glimmering girl" who appeared to him that day when he was out fishing. Even though he's old, he's determined to find out where the girl has gone. He imagines that, when he finally finds her, he'll kiss her and hold her hands. He also says that he and she will walk among the grass, and together they'll pluck the "silver apples of the moon" and the "golden apples of the sun" until the end of time

Explanation: