1. I wanted to compare like quantities, so I determined a common denominator for all the fractions. (the 8, 5, and 10 --> 40)
So,
3/8 = 15/40 (multiply the fraction by 1 which is 5/5)
1/5 = 8/40 (multiply the fraction by 1 which is 8/8)
3/10 = 12/40 (multiply the fraction by 1 which is 4/4)
2. John gave away 15/40 + 8/40 = 23/40.
He threw away 12/40.
3. "He ate the rest" means whatever is left. From Step 2 above, 23/40 + 12/40 = 35/40. Well, 40/40 would be the whole box of plums, right? So, the difference between 40/40 and 35/40 is 5/40. (40-35)
4. What fraction did he give away? 23/40 (from step 2.)
5. What fraction did he eat? 5/40 (or 1/8 reduced) ... (from step 3.)