This is kind of a start at the end question. Wherein you solve the problem backwards. First she had 9 muffins:
1-[tex]\frac{2}{3}=\frac{1}{3}[/tex]
Meaning that 9 is the one-third or the 33.33% of the original, so:
NV(the remaining)=Percentage x Original
9=[tex]\frac{1}{3}[/tex] x Original
Divide both sides by one-third
27=original
now before she gave the two-third of the muffins to her niece, she has 27 pcs left.. but there is another problem, she gave 1/10 of muffins to her children that resulted to 27 pcs left... therefore we do it again:
1-[tex]\frac{1}{10}=\frac{9}{10}[/tex]
meaning 27 is the nine-tenth value of the original so lets do what we did earlier by substituting it in the equation,
NV=Percentage x Original
27=[tex]\frac{1}{10}[/tex] x Original
Divide both sides by one-tenth
Original=30
She had 30 muffins baked before she gave it to the kids and to her nieces. Hope this helps =)