4.NF.B.4.b: Understand a multiple of a∕b as a multiple of 1∕b, and use this understanding to multiply a fraction by a whole number. For example, use a visual fraction model to express 3 x (2∕5) as 6 x (1∕5), recognizing this product as 6∕5. (In general, n x (a∕b) = (n x a)∕b.)