Well, to find the cost of yogurts, you would use
N = Y$0.50 - $0.30
(The cost of yogurts is the price of yogurts times the amount you bought, minus the $0.30 from the total)
If you bought 20, it would be:
0.5 x 20 = 10 10 - 0.3 = $9.70
The answer is the first one, hope this helps :)