The first problem Levi solves is incorrect, because 5/4 is not equal to 20.
The second problem Levi solves is correct, because 1.5 x 10 is 15, which subtracting 8 is equal to 7.
The third problem Levi solves is correct, because 4 x 13 is 52 and plus 6 it is 58.
I hope this helps!