Question

Simplify using mental math 7 / 10 + 2 / 5 - 1 / 10

Answer 1

Common denominator first. multiply 2/5 by 2 in the top and bottom so they all have a denominator of 10

7/10+4/10-1/10=10/10

10/10=1

Answer 2

Assuming mental math still follows the order of operations, you have
$\frac{7}{10} + \frac{4}{10} - \frac{1}{10} = \frac{10}{10} = 1$

So the answer is 1.