Question 212084
Let N be the number of nickels and Q the number of quarters.
You will have two equations. 
The first relates the number of coins.
"he has 15 more nickels than quarters"
1. N=15+Q 
The second is the total value of the coins. 
Each nickel is worth 5 cents. 
Each quarter is worh 25 cents. 
The total is $4.35 or 435 cents.
2. 5*N+25*Q=435. 
Now substitute the expression for N into eq. 2 and solve for Q. 
Then go back to eq. 1 and solve for N. 
Re-post or email if you get stuck or need more help. 
Good luck!