Sonji 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?

To find the number of nickels and dimes, we can use this equation: 0.10d + 0.05n = 5.25

We are given the information that Sonji has 21 dimes (which means that d=21). We can use the equation to find the number of nickels n.

0.10d + 0.05n = 5.25

0.10 (21) + 0.05n = 5.25

2.10 + 0.05n = 5.25

0.05n = 3.15

n = 3.15 / 0.05

n = 315 / 5

n = 63

Sonji has 63 nickels.

Let's check our answer.

21 (0.10) + 63 (0.05) = 2.10 + 3.15 = 5.25

It seems that our answer is correct.