The product of a number b and 3 is no less than 12.

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The product of a number b and 3 is no less than 12.

A number b is just written as b. So we have:
The product of b and 3 is no less than 12.

take this in parts:

• The product of b and 3: 3b
• The phrase is no less than means an inequality, so we have greater than or equal to. We set 3b greater than or equal to 12

3b >= 12