Let d = # of dimes and
    q = # of quarters.

Then you have a system of 2 equations in 2 unknowns

10d + 25q = 205,
10d +  5q = 105.

Distract the second equation from the first one. You will get

20q = 100. Hence, q = 5.

Then d =  = 8.

Answer. q = 5,   d = 8.

Number of dimes: 

Number of quarters:    

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