Define ages based on ratio

Let the ages of Sara and Dipti one year ago be $3x$ and $4x$. Their present ages are $3x+1$ and $4x+1$.

$Sara_{past} = 3x, Dipti_{past} = 4x$

Set up equation for future ages

Four years hence, their ages will be $(3x+1)+4 = 3x+5$ and $(4x+1)+4 = 4x+5$. Given the ratio is $4:5$, we equate:

$\frac{3x+5}{4x+5} = \frac{4}{5}$

Solve for x

Cross-multiplying: $5(3x+5) = 4(4x+5) \Rightarrow 15x + 25 = 16x + 20$. Rearranging gives $x = 5$.

$x = 5$

Calculate present age of Sara

Sara's age one year ago was $3x = 3(5) = 15$. Adding 1 year gives the current age.

$15 + 1 = 16$