The sequence is arithmetic since the common difference that is equal
d=11-15=7-11=-4
a(n) = a1 +(n-1)d
a1=15
=
a(n) = 15+(n-1)-4
Highest 2-digit number = 99
<span>Highest 1-digit number = 9 </span>
<span>Highest possible sum from 2, 2-digit numbers = 198 </span>
<span>Highest possible sum from 2, 1-digit numbers = 18</span>