To add 3/4 and 1/3, we have to make the denominators (the numbers on the bottom) equal.
So, we need to find the LCM, or the least common multiple, of 4 and 3! Let's list some multiples.
3*1=3
3*2=6
3*3=9
3*4=12
3*5=15
4*1=4
4*2=8
4*3=12
4*4=16
4*5=20
The first products (answers to multiplication problems) that 3 and 4 have in common is 12, so the LCM is 12.
We have to get the denominators to equal 12. We can do this by multiplying the denominators.
Let's start with 3/4. 4 times what equals 12? If you look at the multiples we listed above, you will see that 4 times 3 equals 12. So, we have to multiply the denominator by 3... But remember that anything we do to the denominator must also be done to the numerator (the number on top)! We also have to multiply the numerator, 3, by 3.
3*3=9
4*3=12
We now see that 3/4 is equal to 9/12!
Let's change 1/3 next. What times 3 equals 12? 4! Remember to multiply the numberator, 1, by 4, too!
1*4=4
3*4=12
1/3 is equal to 4/12!
Now we can subtract.
We have 9/12-4/12. Remember not to subtract the denominators! They are the "whole" amount, so they don't decrease or increase.
9-4=5. Keep the denominators, 12, the same! Our answer is....
5/12!
Hope I helped! :)