Merci de m'aider. 1/21 + ? = 16/42

Bonsoir,

1/21 + ? = 16/42
16/42 - 1/21 = ?
16/42 - 1 x 2 / 21 x 2 = ?
16/42 - 2/42 = 14/42
Et
14/42 = 14 x 1 / 14 x 3 = 1/3

Donc : 
1/21 + 1/3 = 16/42