(1)12÷(1-20%-25%-45%)
=12÷10%
=120(人)
答:三年级一共有120人.
(2)120×(45%-25%)
=120×20%
=24(人)
答:得B等的人数比得A等的少24人.
(3)(25%-20%)÷20%
=5%÷20%
=25%
答:得B等的人数比得C等的多25%.
故答案为:120,24,25%.
(1)12÷(1-20%-25%-45%)
=12÷10%
=120(人)
答:三年级一共有120人.
(2)120×(45%-25%)
=120×20%
=24(人)
答:得B等的人数比得A等的少24人.
(3)(25%-20%)÷20%
=5%÷20%
=25%
答:得B等的人数比得C等的多25%.
故答案为:120,24,25%.