Dear Mathee2, you can do 1/10+1 8/10≈2 and 1/20+1 19/20≈2. Note: you said fractions, so did you want the fractions to be mixed numbers or improper fractions?
It gets you closer to your answer. Ex: 11×2-- Round 11 to 10 and then multiply and get 20. But if you get some answer like 50, you know it is wrong. But the answer is 23
