わからない個所を教えてください。
‘There’s nothing here,’ he says, sounding frustrated. Then, ‘Wait – I know.’ He looks up at me again. ‘You’ll stay there?’
‘OK.’
‘Won’t be long.’ He turns and runs away across the lawn to the side of the house, and disappears from sight there.
I cling on where I am, wondering what he is going to do. It is like a puzzle, like one of the ones my father gives me to stimulate my mind. （前回の終わりの部分です）
‘Imagine, Alex,’ he says, ‘that you have three pieces of string.’
I try my best, and nod.
‘The first one is as long as two fingers. The second is as long as three fingers, and the third is as long as six fingers. Do you think if you put the first and second end-to-end, they’d be as long as the third?’
I stare at the table, but the words muddle me. String and fingers dance in my head: Lena shows me how to make a cat’s-cradle and pass it from person to person; I stretch string tight and twang it, feeling the quick-dying vibrations in my hands; I coil string tightly around my thumb and leave a red spiral mark when I take it off, neat grooves running all the way up.
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主人公のAlexはartistで、展示会に備えるために、幼い頃過ごしたイタリアに戻ってきています。（この英文は子供時代の回想シーンだと思われます）
Do you think if you put the first and second end-to-end, they’d be as long as the third?’について、この答えは「no」ですか？？
（最初の紐の長さは指２つ分、次のは指３つ分、３番目のは指6つ分なので、最初の紐と次の紐を合わせても3番目の紐の長さにはならない？）
Lena shows me how to make a cat’s-cradle and pass it from person to personのpass it from person to personのところは、あやとりの作り方をLenaと言う人が主人公に示し、人から人へあやとりの紐を渡す（回す）ということでしょうか？
（なぜあやとりをcat’s-cradleというのかがちょっと不思議なのですが）
I stretch string tight and twang it, feeling the quick-dying vibrations in my handsの中の、the quick-dying vibrationsとはどういう意味ですか？
教えてください。お願いします。
補足
なるほど。。。すみませんが、１番目のピースの長さをXとして方程式を作ってみてもらえますか？そのほうが応用がきくと思います。 よろしくお願いします。