Add fractions with unlike denominators when the denominator is 10 or 100. Once practiced, do the same mentally. For example, 3/10 + 2/100 = 32/100. This is similar to adding decimals without regrouping.
Watch these Khan Academy videos:
Do these Khan Academy exercises:
Next, give your students this challenge:
2,4,6,8 by NRICH
Conclude by leading this investigation:
Complete the Quadrilateral
4.NF.C.5: Express a fraction with denominator 10 as an equivalent fraction with denominator 100, and use this technique to add two fractions with respective denominators 10 and 100. For example, express 3/10 as 30/100, and add 3/10 + 4/100 = 34/100.