1. The problem is to find the number that should replace the gap in the equation: $$16 \times 21 = (16 \times 3) \times \square$$ 2. Start by simplifying the left side: $$16 \times 21 = 336$$ 3. Simplify the known part on the right side: $$16 \times 3 = 48$$ 4. Now the equation is: $$336 = 48 \times \square$$ 5. To find the missing number, divide both sides by 48: $$\square = \frac{336}{48} = 7$$ 6. Therefore, the number that should go in the gap is 7.