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.