The speeds A and B are in the ratio 3 : 4. A takes 20 min more than B to reach a destination , In what time does A reach the destination 

Let the time taken by A be $x\text{\hspace{0.17em}hrs}$

Then time taken by  $B=(x-\frac{20}{60})hrs$

$=x-\frac{1}{3}hrs$

$\text{Ratio\hspace{0.17em}of\hspace{0.17em}the\hspace{0.17em}speeds}=\text{Inverse\hspace{0.17em}ratio\hspace{0.17em}of\hspace{0.17em}times}$

$3:4=(x-\frac{1}{3}):x$

$\Rightarrow \frac{x-\frac{1}{3}}{x}=\frac{3}{4}\Rightarrow \frac{3x-1}{3x}=\frac{3}{4}$

$12x-4=9x\Rightarrow 3x=4\Rightarrow x=\frac{4}{3}$

$x=1\frac{1}{3}hrs$