1.
8/10×2/9+2/10×1/9=20%
2.
8/10=80%
3.
8/10×(2/9×1/8+2/9×7/8+7/8×2/9)+2/10×(1/9×8/8+8/8×1/9)=17/45
4.
1-8/10×7/9×6/8=8/15