Question 158322
working in cents


let n=number of coins


"The average value of all the pennies, nickels, dimes and quarters in Bob's pocket is 20 cents" __ 20n


"If he had one more quarter, the average would be 21 cents" __ 20n+25=21(n+1) 


20n+25=21n+21 __ 4=n


4 coins worth 80 cents means 3 quarters and a nickel
__ no pennies or dimes