Mida has $2.25 in nickels and dimes. She has four times as many dimes as nickels. How many dimes and how many nickels does she have?

"$2.25 in nickels and dimes"

n and d are the number of nickels and dimes, respectively

0.05n + 0.10d = 2.25

"four times as many dimes as nickels"

d = 4n

substitution

0.05n + 0.10 (4n) = 2.25

0.05n + 0.40n = 2.25

0.45n = 2.25

n = 2.25/0.45 = 5

5 nickels and 20 dimes

She has 5 nickels and 20 dimes.