I make some rough calculation on the back of an envelope.

我的信封背面大概算了一下.

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The image is of an ill - thought - out idea scribbled on the back of an envelope.

它的形象是未经缜密考虑,潦草地在信封背面上打草稿的 — 个主意.

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I made some rough calculations on the back of an envelope.

我在信封背面做了一些估算.

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