Call the number of quarters Q and the number of dimes D. So the value of the coins is 25Q+10D=1425.
The total coins Q+D=75, so D=75-Q. Substitute for D in the other equation:
25Q+10(75-Q)=1425; 25Q+750-10Q=1425; 15Q=1425-750=675, so Q=675/15=45.
The number of quarters in the jar is 45 (and 30 dimes).