In a bag, there are coins of 25 p, 10 p and 5 p in the ratio of 1 : 2 : 3. If there is Rs. 30 in all, how many 5 p coins are there?

Let the number of 25 p, 10 p and 5 p coins be x, 2x, 3x respectively
Then, sum of their values
$$\eqalign{ & = Rs.\,\left( {\frac{{25x}}{{100}} + \frac{{10 \times 2x}}{{100}} + \frac{{5 \times 3x}}{{100}}} \right) \cr & = Rs.\,\frac{{60x}}{{100}} \cr & \therefore \frac{{60x}}{{100}} = 30 \Leftrightarrow x = \frac{{30 \times 100}}{{60}} = 50 \cr & {\text{Hence,}}\,{\text{the}}\,{\text{number}}\,{\text{of}}\,{\text{5p}}\,{\text{coins}} \cr & = \left( {3 \times 50} \right) \cr & = 150 \cr} $$