You're on a right track.
a + (a+d) + (a+2d) + ...+ (a+9d) = 165 = 10a + 45d = 165
(a+10d) + (a+11d) + (a+12d) + ...+ (a+19d) = 445 =
(a+10d) + (a+10d+d) + (a+10d+2d) + ...+ (a+10d+9d) = 10a + 45d + 100d =
10a + 145d = 445
Now you can solve for a and d if you wish, but you may wish to hold off on that.
Third 10 - You should be able simply to write this one down.
10a + 245d = ??
Of course, there is always the really easy way. This is all linear. Why mess with it?
445 - 165 = 280
?? - 465 = 280