Let's compare apples to apples by finding a common denominator...
2/3
7/8
3×8 = 24 we'll use that
You multiply 3 by 8 to get 24 in the denominator, so multiply the numerator by the same amount...
2/3 × 8/8 = 16/24
Do the same with the other fraction but multiply by 3...
7/8 × 3/3 = 21/24
Now we can compare...
16/24 is less than 21/24 so 2/3 is less than 7/8.