X+Y=2( 18-X-Y)

X-Y= 1/3( 18-X-Y)

Simplified X+Y= 12

2X-Y=9

X=7, Y=5, the third side is 18-7-5=6, and these three sides are 5, 6 and 7.

(2) Passenger cars and freight cars run on two parallel tracks respectively, with the length of passenger cars 150m and the length of freight cars/250m. If two cars are driving in opposite directions, it takes 10 from the moment when the front ends of the two cars meet to the moment when the rear ends leave. If the bus catches up with the truck from behind, it takes 1 min 40 seconds to catch up with the truck from the front of the bus to leave the front of the truck. Find the speed of two cars.

Let the speed of passenger cars be x m/s and the speed of freight cars be y m/s.

10(x+y)= 150+250

100(x-y)= 150+250

X=22 y= 18。

(3) Let the wholesale price of pencils be X and that of erasers be Y..

Then the equation is

2×30×(x+0.05)+30×(y+0. 1)= 30①

3×30×x+2×30×y=40.50 ②

It can be concluded from the above two formulas

X=0.25 yuan

Y=0.3 yuan