math

suppose that 12 inches of wire costs 36 cents.At the same rate, how many inches of wire can be bought for 21 cents?

$\small{ \begin{array}{rcll} \dfrac{ 12\ \text{inches} } { 36\ \text{cents} } \cdot 21\ \text{cents} &=& \dfrac{12}{36}\cdot 21 \ \text{inches} \\\\ &=& \dfrac{1}{3}\cdot 21 \ \text{inches} \\\\ &=& \dfrac{21}{3} \ \text{inches} \\\\ &=& 7 \ \text{inches of wire} \\\\ \end{array} }$

12  = x

36  = 21

Cross multiply

12*21=36x

252=36x

Divide 36 on both sides

7=x

7 inches of wire