Question

How do you solve - 16/21 - (- 13/15)

Answer 1

$\bf -\cfrac{16}{21}-\left( -\cfrac{13}{15} \right)\implies -\cfrac{16}{21}+\cfrac{13}{15}\impliedby \textit{we'll use the LCD of 105} \\\\\\ \cfrac{(-16\cdot 5)~+~(13\cdot 7)}{105}\implies \cfrac{-80~+~91}{105}\implies \cfrac{+11}{105}\implies \cfrac{11}{105}$

in case you wonder why we used 105 as LCD,  

21 = 3 * 7

15 = 3 * 5

the "3" is common to both denominators, so we "use it once" and our LCD is then 3 * 7 * 5.