63+9= - 60+10=, 33+28= - 30+30=, 48+11= - 50+10=, 72-28= - 70-30, 99-44= - 100-40=, 15+62= - 20+60=, 36+44= - 40+40=, 83-72= - 80-70=, 59-19= - 60-20=, 99-77= - 100-80=,

Estimating with addition and subtraction

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