If the number of nickels is n then the number of dimes d is given by the equation n=d/2+6.
So we can work out the number of dimes in terms of n: n-6=d/2, so d=2(n-6).
A nickel is 5c in value and a dime is 10c so n nickels are worth 5n cents and 2(n-6) dimes are worth 20(n-6).
The total worth is 5n+20(n-6)=755 cents.
Expanding the brackets: 5n+20n-120=755. 25n=755+120=875, making n=875/25=35.
So there were 35 nickels (and 58 dimes).