The ratio of Neha's age to that of Jane is 4:7 and the ratio of Jane's age to that of Smita is 4:3. If Smita is 21 years old, how old is Neha?

Ratio of Neha's age to that of Jane is 4:7. This means;
$\frac{\mathrm{Neha}\text{'}\mathrm{s} \mathrm{age}}{\mathrm{Jane}\text{'}\mathrm{s} \mathrm{age}}=\frac{4}{7}$
So, Neha's age = $\frac{4}{7}\times$ Jane's age.........(i)
Now, Ratio of Jane's age to that of Smita is 4:3. This means;
$\frac{\mathrm{Jane}\text{'}\mathrm{s} \mathrm{age}}{\mathrm{Smita}\text{'}\mathrm{s} \mathrm{age}}=\frac{4}{3}$
So, Jane's age = $\frac{4}{3}\times$ Smita's age = $\frac{4}{3}\times 21 = 28$ years
Putting Jane's age in (i) we get;
Neha's age = $\frac{4}{7}\times 28 = 16$ years
Therefore Neha is 16 years old.