Final answer:
To check the expression 1 2/5−7/10, David can convert mixed numbers to improper fractions, find common denominators, and subtract, use a calculator, or apply estimation to ensure the result is reasonable.
Step-by-step explanation:
To check the calculation of the expression 1 2/5−7/10, David can use several methods. One way is to convert the mixed number to an improper fraction and then find a common denominator for both fractions before subtracting. So, converting 1 2/5 to an improper fraction gives us 7/5 (since 1 2/5 = 5/5 + 2/5 = 7/5). Now, having 7/5 and 7/10, we find the common denominator, which is already 10 for both. Now subtract: 7/5 (which is the same as 14/10) minus 7/10 equals 7/10 after the subtraction.
Another way to check his work is to use a calculator to perform the subtraction if allowed. Lastly, David might also use estimation to check that his answer is reasonable; since 1 2/5 is greater than 7/10, his final answer should also be greater than 1.