Đáp án:
` a, 6/25 : 21/20 `
` = 6/25 ` × ` 20/21 `
` = ( 6 × 20 )/( 21 ×21 ) `
` = 8/35 `
` b, 40/7 ` × ` 14/15 `
` = ( 40 × 14 )/( 7 × 15 ) `
` = 16/3 `
` c, 17/13 : 51/26 `
` = 17/13 ` × ` 26/51 `
` = ( 17 × 16 )/( 13 × 51 ) `
` = 2/3 `
` @Akira2007 `