ratios

3 out 5 voted for Tommy. This is equal to 9 out of 15 (copy the ratio three times). So 15 voted in total.

CPhill has continued this here:       (Thanks Chris)

https://web2.0calc.com/questions/help-me-melody_2

I will just copy CPhill's answer over, it is probably easier:

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2 out of 5 students voted for John. The other students voted for Tommy. If 9 students voted for Tommy, how many students voted in all?

In every group  of 5,  2 voted for John and 3 voted for Tommy

So

2/3  of Tommy's vote  = Jonh's vote

(2/3) (9)  =   6  voted for John

9 +  6   =    15   voted