Hi pseries,

The rules of math stay the same, regardless of what the numbers (or variables) are.

When adding (or subtracting) fractions, you need a common denominator:

With your first example:

3/8 + 3/10 has a common denominator of 40 (although you could use 80 and reduce the result later), so you need to convert both fractions:

3(5)/8(5) + 3(4)/10(4)

15/40 + 12/40 = 27/40

With your second example:

1/4 + 3/6 has a common denominator of 12 (you could also use 24 and reduce later), so convert the fractions:

1(3)/4(3) + 3(2)/6(2)

3/12 + 6/12 = 9/12 = 3/4

As you learn (or relearn) more and more math, you have to memorize the concept, then be prepared to use it in ways that are other than obvious (such as with variables, exponents, radicals, etc.).

GMAT assassins aren't born, they're made,
Rich