求未知数X，写出第一步列式的根据．
①3200-X=485
②4872÷X=203．
①3200-X=485，
3200-X+X=485+X，（根据等式的性质）
485+X=3200，
485+X-485=3200-485，
X=2715；
②4872÷X=203
4872÷X×X=203×X，（根据等式的性质）
203X=4872，
203X÷203=4872÷203，
X=24．
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