求小学五年级奥数题几道（过程及答案）

7.5*23+31*2.5=7.5*23+2.5*23+2.5*8=(7.5+2.5)*23+2.5*4*2=230+10*2=250 0.79*0.46+7.9*0.24+11.4*0.079 =0.79*0.46+0.79*1.14+7.9*0.24 =0.79*1.6+7.9*0.24 =7.9*(0.16+0.24) =7.9*0.4 =(8-0.1)*0.4 =3.2-0.04 =3.16 199.199x198-198.198x199 =199*198+0.199*