有红黄两种颜色的小球共140个，拿出红球的1/4，再拿出7个黄球，剩下的红球和黄球正好－样多。原来红球和黄球各有多少个？

数学题解答：题中有红黄两种小球，且告诉了它们的总数，要求两种小球各有多少个。另外，题中还有“剩下的红球与黄球一样多”的等量关系，所以用方程解答比较容易。

解：设黄球有x个，则红球有（140-x）个。

红球拿起了1/4，还剩下3/4，所以剩下的红球可表示为：(140-x)×（1-1/4），剩下的黄球可表示为x-7，它们是相等的关系。列方程为：

(140-x)×（1-1/4）＝x-7

（140-x）×3/4        =x-7

                140×3/4  -3/4x=x-7

                                 7/4 x-7=105

                                       7/4x=105+7

                                      7/4x=112

                                               x=64

黄球有64个，红球有：140-64＝76（个）。

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