Let h, q, and d denote the number of half-dollars, quarters and dimes.

Then what you are told is:

h + q + d = 70,

50*h + 25*q + 10*d = 1775

and:

q = 3*d

Substitute the last of these into the first two which will give you a pair

of simultaneous equations in two variables which you then need to solve.

The attachment shows QuickMath's solution

RonL