?$解:(1)設(shè)兩校人數(shù)之和為\ a .\ $?
?$若\ a\gt 200 , 則\ a=18000 \div 75=240;$?
?$若\ 100\lt a \leqslant 200 , 則\ a=18000 \div 85=\ \ 211 \frac{13}{17} , 不合題意.\ $?
?$所以這兩所學(xué)校報(bào)名參加旅游的學(xué)生人數(shù)之和等于 240 人, 超過(guò) 200 人.\ $?
?$(2)設(shè)甲校報(bào)名參加旅游的學(xué)生有\(zhòng) x\ 人, 乙校報(bào)名參加旅游的學(xué)生有\(zhòng) y 人.\ $?
?$①當(dāng)\ 100\lt x \leqslant 200\ 時(shí), 得\ \left\{\begin{array}{l}x+y=240, \\ 85 x+90 y=20800 .\end{array}\right.\ 解得\ \left\{\begin{array}{l}x=160, \\ y=80 .\end{array}\right.\ \ $?
?$②當(dāng)\ x\gt 200\ 時(shí),得\ \left\{\begin{array}{l}x+y=240, \\ 75 x+90 y=20800 .\end{array}\right.\ 解得\ \left\{\begin{array}{l}x=53 \frac{1}{3}, \\ y=186 \frac{2}{3} .\end{array}\right.\ \ $?
?$此解不合題意, 舍去.$?
