Question

Four students are running for class president: liz, sam, sue, and tom. the probabilities of sam, sue, and tom winning are 7⁄25 , 3⁄10 and 21⁄100 , respectively. what is the probability of liz winning?

Answer 1

First, we need to convert all the fractions to have the same denominator. 10 and 25 are both multiples of 100, so 100 would be appropriate. 

Sam has a 7/25 chance. Because we want ?/100, something needs to change. To get from 25 to 100, you need to times 25 by 4, right? So, do the same with the 7.
7 x 4 = 28. Therefore Sam has a 28/100 chance.

Sue has 3/10. Using the same method, we can see that 3 needs to be multiplied by 10 (because 10 times 10 = 100). So Sue has a 30/100 chance.

Tom is already in the fraction we like, so just keep this as 21/100. 

Now, add 28/100, 30/100 and 21/100 to get 79/100. 

Because won of them will get the role of class president, we know that the probability adds to 1. To get a full probability (100/100, or 1), what needs to be added to 79/100? 

Another way of going about this is 100/100-79/100. The answer is 21/100

The probability of Liz winning is 21/100. 

Let me know if this is still unclear, I would be more than happy to explain in more detail if necessary :)