If the height of the tree that casts a shadow of 10 m is 14 m, then find the height of a tree that casts a shadow of 15 m. 

The more the height of the tree the greater will be its shadow, therefore, it is a case of direct proportion.

So, $\frac{{\mathrm{x}}_1}{{\mathrm{y}}_1}=\frac{{\mathrm{x}}_2}{{\mathrm{y}}_2}$

Here, ${\mathrm{x}}_1=14, {\mathrm{y}}_1=10, {\mathrm{x}}_2=? \mathrm{and} {\mathrm{y}}_2=15$

=> $\frac{14}{10}=\frac{{\mathrm{x}}_2}{15}$

=> ${\mathrm{x}}_2=\frac{210}{10}$

=> ${\mathrm{x}}_2=21 \mathrm{m}$ 

Hence, the height of the tree is 21 m.