25203

I used anna's method to help me. I started trying to put them together then I found that there was lots of gaps. So then I began with 18 squares across (which was the biggest). Then I worked along the outside to calculate the numbers that would go together and wouldn't leave any gaps. For example I put the 15 next to the 18 then I worked out that I needed 7 and 8 above the 15 because 7 add 8 equales 15, then 4 filled the gap