填一填．
70+70+70+70= × =
51×4= × + × = + =
700+700+700= × =
3×400= + + =
× =90+90+90+90= ．
填一填．
70+70+70+70= × =
51×4= × + × = + =
700+700+700= × =
3×400= + + =
× =90+90+90+90= ．
的分母加上21后，要使分数的大小不变，分子应 ．
的分子加上15，分母加上24，分数的大小不变． ．
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