It is not written very well on the service sheet. Go back to where it says "How to calculate"

That is a fraction.

The next bit "The above represents" It is also a fraction, the numerator, the top bit is written correctly as 12 x 30 but the denominator, the bottom bit is written incorrectly and should read 12 + 3 (not 12 x 3). They have also put in a second equals sign which makes it even more confusing, so ignore that second equals sign and you get the fraction 360/15=24.