Replace `square` in each of the following by the correct number :
(i) `1/3=square/21" (ii) "45/60=3/square`

Answer

To solve the given problems, we will replace the `square` with the correct number by using cross multiplication. Let's break it down step by step.

### Question (i):
**Find `square` in the equation:**
\[ \frac{1}{3} = \frac{\text{square}}{21} \]

**Step 1:** Let the `square` be represented by \( x \). So, we rewrite the equation as:
\[ \frac{1}{3} = \frac{x}{21} \]

**Step 2:** Cross multiply to eliminate the fractions:
\[ 1 \times 21 = 3 \times x \]
This simplifies to:
\[ 21 = 3x \]

**Step 3:** To solve for \( x \), divide both sides by 3:
\[ x = \frac{21}{3} \]

**Step 4:** Calculate \( \frac{21}{3} \):
\[ x = 7 \]

So, the value of `square` in the first equation is **7**.

### Question (ii):
**Find `square` in the equation:**
\[ \frac{45}{60} = \frac{3}{\text{square}} \]

**Step 1:** Let the `square` be represented by \( y \). So, we rewrite the equation as:
\[ \frac{45}{60} = \frac{3}{y} \]

**Step 2:** Cross multiply to eliminate the fractions:
\[ 45y = 3 \times 60 \]
This simplifies to:
\[ 45y = 180 \]

**Step 3:** To solve for \( y \), divide both sides by 45:
\[ y = \frac{180}{45} \]

**Step 4:** Calculate \( \frac{180}{45} \):
\[ y = 4 \]

So, the value of `square` in the second equation is **4**.

### Final Answers:
- For the first question, `square` = 7
- For the second question, `square` = 4