Explanation:
there is nothing "following" here.
however, if I understand correctly, we need a 15% increase to 60 kV.
100% = 60 kV
1% = 100%/100 = 60/100 = 0.6 kV
15% = 1% × 15 = 0.6 × 15 = 9 kV
so, adding these 15% gives us
60 + 9 = 69 kV.
once we understand this mechanism, we can also use the shortcut : to add x% to something, we need to multiply this something by 1.x.
in our case, we want to increase 60kV by 15% ?
the result is
60 × 1.15 = 69 kV
simply because (as the top part should have shown too) x% of the whole = x/100 × whole.
so, to add x% means to have
100/100 × whole + x/100 × whole = 1.x × whole