How do you solve a problem (fraction +/- fraction) by first converting the denominators to their LCM?

Question

The problem is:

23/72 + 12/90

prime factors of 72 = 5, 5, 3 & 3 = 3^2*2^2

prime factors of 90 = 5, 5, 3 & 3 = 5^2*3^2

so the LCM (Lowest Common Multiple) is 5^2*3^2*2^2=1800 (new denominator)

Now, how do I figure out the numerators?

Answer 1

problem and solution fraction