Question 159355
Let N = number of nickels and Q = number of quarters.
1) N = 2Q "I have twice as many nickels as quarters"
2) 0.05N+0.25Q = 4.90 "...the coins are worth $4.90,..."
Substitute the N = 2Q into equation 2) and solve for Q to get the number of quarters.
0.05(2Q)+0.25Q = 4.90 Simplify.
0.35Q = 4.90 Divide both sides by 0.35
Q = 14 This is the number of quarters.
N = 2Q = 2(14) = 28  This is the number of nickels.
Check: 
28(0.05)+14(0.25) = 1.40+3.50 = 4.90