Question

how did the circled step come​

Answer 1

Since we have:

$y + \frac{20y}{100} = 18$

If we want to add the first y into the fraction, we must make the denominators equivalent

Currently we have:
$\frac{y}{1} + \frac{20y}{100} = 18$

To turn 1 into 100, we need to multiply it by 100, so we multiply by 100 in both numerator and denominator

$\frac{y*100}{1*100} + \frac{20y}{100} = 18$

So, we get:

$\frac{100y}{100} + \frac{20y}{100} = 18$

Now that the denominators are the same, we can add the numerators and keep the denominator constant:

$\frac{100y + 20y}{100} = 18$

I hope you understood this explanation of the circled step!

Answer 2

I hope it helps you Thankyou