口算.
5+7= 14+4= 5–4= 18–8= 15+4=
15–10= 6–3= 7+7= 16–2= 11+7=
2+8= 4+9= 19–10= 4+7= 8+8=
10+9= 12+1= 18–15= 19–1= 4+4=
16–1= 9+7= 4–1= 3+8= 14–2=
18+1= 12–2= 11+6= 10–2= 2+2=
3–2= 18–3= 10+4= 17–3= 9–3=
8–3= 15+1= 2+9= 8+5= 9–4=
4+2= 8–4= 12+2= 4–4= 5+2=
19–5= 15+2= 10–5= 7–5= 0+5=
9+9= 19–6= 0+9= 10–6= 9–6=
4+1= 6–6= 18–7= 9+6= 8–7=