(12) A man has saved $640 during the first month,$720 in the second month and $800 in the third month. If he continues his savings in this sequence, what will be his savings in the 25th month?

Solution:
If we write his saving like a sequence we will get 640,720,800,………… to get 25th month savings we have to find the 25th term of the sequence
a = 640   d = t2 – t1
              = 720- 640
              = 80
tn = a + (n-1) d
t25 = 640 + (25 - 1) 80
  = 640 + 24 (80)
= 640 + 1920
= 2560

Therefore he is saving 2560 in the 25th month.