I'm struggling with these two qusetions and would appreciate some help.

7. The dimensions of a cuboid are X x Y x Z, where X is length, Y is width and Z is height. The Volume of the cubiod is 200cm3 and its length is twice its width.

a/Write down 3 equations involving X,Y and Z.

I've got these equations, can someone check that i have the correct answers please. 2Y=X , 3YZ=200 , 2ZY+2XY+2ZX=240.

I have no idea how to do this question so any help will be appreciated.

8. £1,000 is invested in a bank account on 1st Jan 1994. A further £1,000 pounds is invested at the start of each year, the final pament was on 1st Jan 2003. When the account was closed on 1st Jan 2004 it was worth £15,000.

a/ Annual interest rate is p% and r=1+p/100. Show that r + r2 + ...+ 16r = 15

Thanks.