Step one you make the a SSUMPTION that 120 is 100% since it is your output value, step to you next represent the value you seek with X yes, step three form step one, it follows that hundred percent equals 120, step four in the same VEIN, X percent equals 78, step five it gives you a pair of simple equations like 100% equals 120 (1)
X percent equals 78(2)
And step six you simply divide equation one by equation 2 and taking note of the fact that both the LHS (left-hand side) of both equations have the saying unit( percentage)
Therefore, 78 is 65% of 120
I hope this really helps...