2x+2x=24
x=24÷4
x=6
-----------------
<span>а)(3/4+1/6)+7/12=3/4+(1/6+7/12);
</span>(3/4+1/6)+7/12 =
3/4+(1/6+7/12) =
=
<span>
б)7/15+(2/9+5/6)=(7/15+2/9)+5/6.
</span>7/15+(2/9+5/6)=
=
(7/15+2/9)+5/6 =
=
1) 12 + 5 = 17 (кус) - посадят за час обе бригады
2) 17 * 3 = 51