We can write a fraction to represent how many students passed.
There are #45# students in total but only #18# out of #45# students passed. The key phrase there was "out of"
The phrase #18# "out of" #45# can be mathematically expressed as
#18/45#
This results in a decimal, #0.4# but we want to know what percentage of the students passed. To find this percentage, we multiply the decimal by #100# and add a #%# sign.
Therefore,
#0.4*100%=40%#
So we can conclude that #40%# of the class passed the biology test which also means #60%# did not.