How many seconds are there in 3/4 of a minute?

For this case the first thing you should know is the following conversion:
 
 We can then make the following rule of three:
 1 minute ----------> 60 seconds
 3/4 minutes -------> x
 From here, we clear the value of x.
 We have then:
 
 
 there are 45 senconds in 3/4 of a minute