An electrician charges Mr. Speer $300 before sales tax, for wiring his home. (a) What is the amount of sales tax that Mr. Speer has to pay? (b) What is the total amount Mr. Speer has to pay?

This is asking which 2 paired items have the same fractional ratio I believe. So all we have to do is make ratios of price to item amount and see if they are equal.
1. 4.5/3 = 6/5
this one we divide numerator and denominator of left side by 1.5 and get:
3/2 = 6/5
this is not true so wrong

2. 5/2 = 15.25/6
2.5 = 2.5416666
sometimes easier to use calculator then reduce. But not equal so wrong.

3. 4.75/5 = 9/10
95/100 = 9/10
wrong

4. 9/4 = 13.5/ 6
4.5/2 =4.5/2
this one is equal