a) $20+79=99$  

$205+41=246$

$83-51=32$

$127-67=60$

$435-215=220$

b)`4*21=84

$5\cdot 6\cdot 7=210$

$400:8=50$

$720:9=80$

$255:5=51$

c) $24\cdot 100=2400$  

$170\cdot 10=1700$

$60\cdot 200=12000$

$40\cdot 1100=44000$

$640:80=80$