Onji has $5.25 in nickels and dimes. the coin value can be modeled by the equation 0.10d + 0.05n = 5.25, where d represents the number of dimes and n represents the number of nickels. if sonji has 21 dimes, how many nickels does she have

This is how you solve this problem:
.10d + .05n = 5.25

(plug in the number of dimes we have)

(21 x .1) + .05n = 5.25

(multiply number of dimes x value of the dime)

2.10 + .05n = 5.25

(subtract 2.10 from both sides)

.05n = 3.15

(divide both sides by .05 to get the number of nickels)

n = 63 nickels

Hope this helps!