KMM's Q is wonderfer.I tellyou the detail.

A={a1,a2,a3,a4},a1<a2<a3<a4. we have a3+a4=k1*(a1+a2),a2+a4=k2*(a1+a3),a1+a4=a2+a3,Wehave

k1>k2>1.Cancel a4,a3 ,(3+k2+k1-k1k2)a2=(k1k2-k1+3k2+1)a1,only k2=2,k1=3 and k2=2,k1=4.we

get a2=5a1,a3=7a1,a4=11a1 and a2=11a1,a3=19a1,a4=29a1.

So,A={a1,5a1,7a1,11a1} A={a1,11a1,19a1,29a1} a1 cantake any whole number.

For EX,   {5,25,35,55}...OK