do you know how to do this?

题目

do you know how to do this?
a purse contain a collection of quarters,dimes,nickel,and pennies.The average value of the coins in the pure is 17 cents.If a penny is removed from the purse,the average value of the coins becomes 18 cents.How many nickels are in the purse?
can you help me to do this question?about math

答案

设:钱包里有X个硬币(SUPPOSE:there are X coins in the purse)(17x-1)/(x-1)=18x=1717*17=289 4 pennies 289-4=285 17-4=1325*11+10*0+5*2=28511+0+2=13there are 2 nickles in the purse