Robert needs to cut four shelves from a board that is 2.5 meters long. The second shelf is 18 centimeters longer than twice the length of the first shelf. The third shelf is 12 centimeters shorter than the first shelf. The remaining shelf is 4 centimeters longer than the first shelf. If Robert must use the entire 2.5 meter board for the shelves, what is the length of the second shelf, in centimeters?

First convert the length of the board into centimeter:
2.5m=250cm,because 1m=100cm
x-use the unknown sign for the first shelf
(x*2)+18-As the second shelf is 18cm longer than twice the length of the first one
x-12-as the third shelf is 12cm shorter than the first shelf
x+4-as the remaining(forth) shelf is 4cm longer than the first shelf
Add up all terms and the result is 250 on the other side of equal sign,solve it:
x+(2x+18)+(x-12)+(x+4)=250
transfer like terms on one side and the other ones on the other side
x+2x+x+x=250-18+12-4
add the like terms up
5x=240
divide 240 with 5 as it is transferred on the other side
x=240/5
finally-the result for the first shelf
x=48
apply the result of the first shelf to get the result of the second shelf
(48*2)+18=96+18=114
I know that it is too long,but I hope that it will help you.. :-)