a)

$25+75=100$

$80-44=36$

$100-76=24$

$8\cdot 70=560$

$304\cdot 2=608$

$32:8=4$

$250:2=125$

b)

$560+330=890$

$1000-650=350$

$340-150=190$

$10\cdot 97=970$

$31\cdot 20=620$

$1500:30=50$

c)

$3500+4500=8000$

$10000-7500=2500$

$6600-4500=2100$

$30\cdot 300=9000$

$3600:40=90$