Question 175619
Call the number of pennies P.  Then the number of nickels is 5P and the number of quarters is P+6.  The dollar amount of pennies is 0.01P, nickels is 0.05(5P) and quarters is 0.25(P+6).  Thus we have

0.01P + 0.05(5P) + 0.25(P+6) = 2.52
0.01P + 0.25P + 0.25P + 1.50 = 2.52
0.51P + 1.50 = 2.52
0.51P = 1.02
P = 2
Thus Bob has 2 pennies.