Problem
Sold and eaten are both gone, so add them over twentieths: 15/20 + 4/20 = 19/20.
Fifth graders rewrite fractions with a common denominator so the fourths and fifths are measured in the same equal pieces, twentieths, and can be added.
5.NF.A.1Identify SubproblemsThe whole is 1, which is 20/20. Take away the 19/20 that is gone to find what is left over.
Thinking of the whole as 20 equal twentieths makes the subtraction a simple count: 20 pieces minus 19 pieces leaves 1 piece.
5.NF.A.2Identify SubproblemsThe tangerines left over are 1/20 of the original amount.
Every tangerine is either sold, eaten, or still left, so those three parts together make up the whole stand.
The whole is 1, so taking the sold part and the eaten part away from it leaves exactly the part still there.
In twentieths the leftover 1/20 is one piece while 1/10 is 2/20, two pieces.
Putting both fractions over the same denominator turns the comparison into counting pieces: 1 piece is fewer than 2 pieces, so the leftover really is less than 1/10.
5.NF.A.2Draw A DiagramMake every part share the same denominator, then it is just counting equal pieces: 1 piece left out of 20 is less than 2 pieces, so less than 1/10.