Réponse :

0.8 = ....../25  ⇔ 0.8 = a/25  ⇔ a = 25 x 0.8 = 20  donc  0.8 = 20/25

1.8 = 27/.....  ⇔ 1.8 = 27/b ⇔ b = 27/1.8 = 15  donc 1.8 = 27/15

0.4 x ...../...... = 1/3 ⇔ 0.4 x  a/b = 1/3 ⇔ b = 1.2 a donc 0.4 x a/1.2a = 1/3

⇔ 0.4 x 10/12 = 1/3

6+4/7 = 42/7+4/7 = 46/7

13 - 5/4 = 52/4 - 5/4 = 47/4

4/7 + 12/14 = 4/7 + 6/7 = 10/7

3/5 + 0.9 = 3/5 + 9/10 = 6/10+9/10 = 15/10 = 3/2

tu fais le reste

Explications étape par étape