(c) Salary of Seema = ₹3000 Savings for 1st three months = 900 Saving for next months = 350, 400, 450.... Now, 900 + 350 + 400 + 450 + ....n terms = 11400 350 + 400 + 450 + ..... + upto n terms = 10500 It is an A.P where a = 350 and d = 50
n
2
[2×350+(n−1)50]=10500 n[650+50n]=21000 50n2+650n−21000=0 or n2+13n−420=0 n2−28n−15n−420=0 (n+28)(n−15)=0 n=15 as n can't be negative So total months = 15 + 3 = 18