How do you simplify 5 3/4*2 1/2 to the lowest number

First convert your mixed numbers to improper fractions.
You do this by multiplying the denominator and the whole number, and then adding the numerator. The denominator remains the same though.

5 3/4 becomes 23/4
2 1/2 becomes 5/2

23/4 x 5/2

When multiplying fractions, just multiply straight across.

23 x 5 = 115
4 x 2 = 8

115/8 is the answer.

Or in mixed number form, divide 115 by 8 and you get

14 3/8