Tick (✓) the correct answer


Tick (✓) the correct answer:

The ages of A and B are in the ratio 5 : 7. Four years from now the ratio of their ages will be 3 : 4. The present age of B is

(a) 20 years

(b) 28 years

(c) 15 years

(d) 21 years


(b) 28 years

Let $\mathrm{x}$ be the common multiple of the ages of $\mathrm{A}$ and $\mathrm{B}$.

Then. the age $s$ of $\mathrm{A}$ and $\mathrm{B}$ would be $5 \mathrm{x}$ and $7 \mathrm{x}$, respectively.

$\therefore \frac{5 x+4}{7 x+4}=\frac{3}{4}$

$\Rightarrow 4(5 x+4)=3(7 x+4)$

$\Rightarrow 20 x+16=21 x+12$

$\Rightarrow 16-12=21 x-20 x$

$\Rightarrow 4=x$

$\Rightarrow x=4$

$\therefore$ Age of $B=7(x)=7 \times 4$

$=28$ years



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