能简算的要简算．
15.6﹣8.9+5.6            7.02+3.8+3.98+2.2        68.4﹣（4.7+8.4）
35.8﹣（40﹣44.2）       100﹣13.72﹣36.82        47.35+16.88﹣7.35．