If you multiply/divide the numerator and denominator of a fraction by the same number, you will get an equivalent fraction.
For the first one, 6 is in the new numerator. Let's divide this by the original numerator to see what they did.
30/6 = 5
So they divided the numerator by 5. To get an equivalent fraction, we must do the same to the denominator.
35/5 = 7
So our new equivalent fraction is 6/7.
For the second one, the new denominator is 5. Let's see what they did to get this.
40/5 = 8
So they divided the denominator by 8 to get 5, let's do the same to the numerator:
8/8=1
So we have 1/5.
For the last one, they have 4 in the new denominator. Let's see what they did to get this denominator:
20/4 = 5
So they divided the denominator by 5, let's do the same thing to the numerator:
15/5 = 3
So we have 3/4.