A firm ordered 15 premium machines and some standard machines. The price of a premium machine was 6 times that of a standard machine. By mistake, the number of premium and standard machines was interchanged in the delivery, which increased the total bill by 62.5%. What was the ratio of the number of premium machines to standard machines in the original order?

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1. Let standard price be $s$. Then premium price is $6s$. 
2. If there are $n$ standard machines, original cost = $15	imes6s + n	imes s = (90+n)s$. 
3. After interchange, cost = $15s + n	imes6s = (15+6n)s$. This is $62.5\% = \frac{13}{8}$ more: $15+6n=\frac{13}{8}(90+n)$. 
4. Solve: $8(15+6n)=13(90+n)\Rightarrow35n=1050\Rightarrow n=30$. 
5. Ratio premium:standard = $15:30 = 1:2$. 
FINAL_ANSWER: c

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