①(4/5+1/6-4/15)×30 =4/5 *30 + 30*1/6 -30 *4/15 =24+5-8 =21 ②(1/2+1/4)÷(2/5-1/10) =3/4 ÷ 3/10 = 10÷4=2.5 X/3 -1/3=1, X/3=4/3 X=4