Question 833437
A + B + C = 405
A = B + 117 (The first plane has 117 more seats than the 2nd plane.)
C = B - 18 (The  third plane has 18 fewer seats than the the second plane.)
Go back to the first equation, but substitute the values
(B+117) + B + (B-18) = 405
Add the Bs
3B +117 -18 = 405
3B +99 = 405
Subtract 99 from each side
3B = 306
Divide each side by 3
B = 102
If the second plane has 102 seats, you should be able to figure out the first and third planes :-)