请教各位XDJM：

OG11怎么没有解释，还有下面几道数学题，请高手解答，谢谢了！

OG11第128题：

if money is invested at r percent interest, compounded annually , the amount of the investment will double in approximately 70/r years, if Pat’s parents invested $5000 in a long-term bond that pays 8 percent interest, compounded annually, what will be the approximate total amount of the investment 18 years later,when Pat is ready for college?

Answer20000

请问怎么算出来的 

OG11第129题：

on a recent trip, Cindy drove her car 290 miles, rounded to the nearest 10 miles, and used 12 gallons of gasoline, rounded to the nearest gallon. The actual number of miles per gallon that Cindy’s car got on this trip must have been between

Answer : 285/12.5 and 295/11.5

请问怎么算出来的

OG11第138题：

At a loading dock,each worker on the night crew loaded 3/4 as many boxes as each worker on the day crew. If the night crew has 4/5 as many workers as the day crew, what fraction of all the boxes loaded by the two crews did the day crew load?

Answer :5/8

请问怎么算出来的

128题

70/r年就可以DOUBLE,所以r=8时,70/8约等于9,也就是说$5000经过9年就可以变为$10000,由此,经过18年,约为$20000