Total money required =Rs.30
On first day money save =Rs.1
Second day =Rs.1.5
Third day =Rs.2
Fourth day =Rs.2.5
We observe the above pattern is AP
with a=1 d=0.5
Now S≥30 (30 is min. amount)
∴S=n2(2a+(n−1)d) formula where n= no. of days
∴n2(2a+(n−1)d)≥30
∴n2(2×1+(n−1)12)≥30
∴n2(n2+32)≥30
∴n(n+3)≥120
∴n2+3n−120≥0
∴n≥−3±√9+4802
∴n≥−3+√4892 Positive avalue
∴n≥19.112;n≥9.55
∴n=10 Nearest integer value
Hence on 10th day she will have just enough money.