I don't know how to write in LaTeX in web2.0calc.com, so I'm going to try without it.

Set variables for the number of dimes and the number of quarters. The number of dimes can be d, and the number of quarters can be q. We already know there are TWICE the amount of dimes than quarters. So, we can say d=2q. since they have cost, we must multiply d or 2q by 10, and q by 25. My work is below:

20q+25q=630(in cents!!!!!) 45q=630(I used a calculator to do 630/45) q=14. 2q=28.

Check: 20(14)+25(14)=630(I used a calculator to do 25•14) 280+350=630. 630=630.

So, Jack has 28 dimes. :)