Mary’s salaries are in A.P. with the first term 15,000 and common difference 100.
\(\therefore\) a = 15000, d = 100, n = 20
Now, tn = a + (n – 1) d
\(\therefore\) t20 = 15000 + (20 – 1) 100
= 15000 + 19 x 100
= 15000 + 1900
= 16900
\(\therefore\) Mary’s salary after 20 months will be Rs.16,900.