蜘蛛有8条腿,蜻蜓有6条腿和2对翅膀,蝉有6条腿和一对翅膀
------------------------------------------------------------------------------------------------------------------不用x
蜘蛛:(118-6*18)/(8-6)=10/2=5(只)
蜻蜓:(20-1*(18-5))/(2-1)=7/1=7(只)
蝉 :(18-5)-7=13-7=6 (只)
------------------------------------------------------------------------------------------------------------------只用x
设蜘蛛有x只
8x+6*(18-x)=118
2x+108=118
x=5
蜘蛛有5只
再设蜻蜓有x只
2*x+1*(18-5-x)=20
x+13=20
x=7
蜻蜓有7只
蝉有18-5-7=6只