如下图所示的梯形的上底是20 cm，下底是34 cm，其中阴影部分的面积是340 $c{m}^2$ ，求这个梯形的面积是____ $c{m}^2$ .

任务型阅读

　　There are some students in front of the lost and found box. The duty monitor Alan is asking some questions.

Alan: Jim, is this your English book?

Jim: No, it isn't.

Alan: Linda, is this your English book?

Linda: Yes, it is. Thanks.

Alan: Gina, what colour is your pencil?

Gina: It's green.

Alan: Is this your pencil?

Gina: Yes, it is. Thank you.

Alan: What is your name?

Jenny: My name is Jenny.

Alan: Is your ruler green or yellow?

Jenny: It's green. There’s my name “Jenny” on it.

Alan: Oh, here you are.

Alan: Here’s a nice bag; whose bag is this?

Paul: Let me see. . .  Oh, it's mine.

Alan: Look at the cute doll. Is this yours, Mary?

Mary: No, it isn't. I think it’s Betty's. Is there a watch in it?

Alan: Sorry, I haven’t seen it so far.

根据对话内容, 完成以下任务。

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