The ratio of urea and potash in a mixed fertilizer is 7 : 3. Express the quantity of urea present as percentage of the total amount of fertilizer.

### Concept & Ratio to Percentage To convert a part of a ratio into a percentage of the whole, you must first find the total number of parts in the ratio. Then, divide the specific part by the total parts and multiply by $100$. $$ \text{Percentage} = \left( \frac{\text{Part}}{\text{Total Parts}} \right) \times 100 $$ ### Step-by-Step Solution 1. Identify the ratio of urea to potash: $7 : 3$. 2. Calculate the total number of parts in the mixture: $$ \text{Total Parts} = 7 \text{ (urea)} + 3 \text{ (potash)} = 10 \text{ parts} $$ 3. Identify the number of parts that represent urea, which is $7$. 4. Calculate the percentage of urea in the total mixture: $$ \text{Percentage of Urea} = \left(\frac{7}{10}\right) \times 100 $$ $$ \text{Percentage of Urea} = 0.7 \times 100 = 70\% $$ ### Exam Strategy & Shortcut Since the total parts sum up to exactly $10$ ($7 + 3$), you can instantly scale this to a percentage (out of $100$) by multiplying both parts by $10$. The ratio $7 : 3$ becomes $70 : 30$ out of $100$. Thus, urea is clearly $70\%$. ### Common Pitfall Students sometimes calculate the percentage based on the other component, e.g., $\frac{7}{3}$, instead of the total mixture. Always divide by the sum of the ratio parts when asked for a percentage of the "total amount". ### Final Answer Therefore, the correct answer is **70%**.