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Ответ:

(3а/5b ÷ 4a/7b) × 20/21
(3а/5b × 7b/4a) × 20/21
( 3/5 × 7/4 ) × 20/21
21/20 × 20/21 = 1

( 25 ху/ 3аb × 24 b²/ 35x²y) ÷ 1/х
( 5/а × 8b/7х) × х
40b/7ax × x
40b/7а