Trying to understand example problem on how to solve rates

[Needhelppleasehelp]

Im studying these notes on how to solve rates. I dont understand how it's telling me to calculate it. Can someone explain where the 120 comes from in the screenshot?

[phw]

Many people prefer working with whole numbers to fractions and when confronted with an equation involving fractions will try to get rid of them as soon as practical -- typically by multiplying both sides of the equation by the least common multiple of everything that appears in a denominator. In this case 120 is the LCM of 40 and 60 so multiplying by it converts an ugly fractions equation into a nice easy whole numbers equation.