If all were dimes, Kelly would have $2.40. She has $1.20 more than that. Each quarter that replaces a dime adds $0.15 to the total, so she must have 1.20/0.15 = 8 quarters.
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Check
.. 8 quarters = $2.00
.. 16 dimes = $1.60
total = 24 coins = $3.60