27+51 = 30+50 = 80
17+39=20+40=60
48+25=50+30=80
52-15=50-20=30
43-19=40-20=20
28+19=30+20=50
19+45=20+50=70
79-18=80-20=60
69-22=70-20=50
55+33=60+30=90