Question 157929
Let's call the pennies P and quarter Q.
Each penny is worth $0.01 and each quarter $0.25.
Total number of coins is 31.
1.P+Q=31
Total value of coins is $5.11.
2.$0.01*P+$0.25*Q=$5.11
Let's multiply both sides of eq. 2 by 100 to get rid of decimals.
2.{{{P+25Q=511}}}
From eq. 2, 
2.{{{P+25Q=511}}}
{{{P=511-25Q}}}
Substitute this expression into eq. 1 and solve for Q.
1.{{{P+Q=31}}}
{{{(511-25Q)+Q=31}}}
{{{-24Q=31-511}}}
{{{-24Q=-480}}}
{{{highlight(Q)=20}}}
from above
{{{P=511-25Q}}}
{{{P=511-25(20)}}}
{{{P=511-500}}}
{{{highlight(P)=11}}}
11 pennies abd 20 quarters.