To add these two fractions, you must first get them having common denominators. Notice that the denominators are factors of each other. 4*25=100. So you can multiply the first fraction by 4/4 and get common denominators.
(7/25)*(4/4) + 3/100
And when multiplying fractions, you just multiply across so your new fractions are:
28/100 + 3/100
Now that you have common denominators, you can add the tops together.
28/100 + 3/100 = 31/100
31 is a prime number so it can't be simplified so you are done!