Answer:
8.21 months.
Step-by-step explanation:
The growth rate of hair, R = 15 cm/year
As 1 year = 12 months,
[tex]\Rightarrow R=\frac{15}{12}[/tex] cm/month
As 1 cm ≈ 0.39 in.,
[tex]\Rightarrow R= \frac{15}{12}\times 0.39=0.4875[/tex] inch/month.
As she wants to grow it 4 inches longer than it now, So, the time required for 4 inches growth = 4/0.4875=8.21 months.
Hence, she needs to wait for 8.21 months.