I assume you are asking one of three things, so I will answer all of them.
if you are asking what is 1/2 of 312, then multiply 312 by 1/2. 312 is really just 312/1, so
we multiply fractions by multiplying straight across
if you were asking what part of 3 1/2 is 1/2 then we have to recognize what 3 1/2 is as an improper fraction. to do this, we convert 3 into halves. how many halves of pizza are there in 3 whole pizzas? 6. so we have 6 halves and one half, which is a total of 7 halves.
now, we do the same thing as the first problem
if you were asking what is half of 3.12, then we convert 1/2 to a decimal and multiply
if you are asking what is 1/2 of 312, then multiply 312 by 1/2. 312 is really just 312/1, so
we multiply fractions by multiplying straight across
if you were asking what part of 3 1/2 is 1/2 then we have to recognize what 3 1/2 is as an improper fraction. to do this, we convert 3 into halves. how many halves of pizza are there in 3 whole pizzas? 6. so we have 6 halves and one half, which is a total of 7 halves.
now, we do the same thing as the first problem
if you were asking what is half of 3.12, then we convert 1/2 to a decimal and multiply