Question 451516
Let's use the following:

t - number of 10 cents stamps
f - number of 15 cents stamps

Everything has to be expressed in cents.

Money Christie used is       -> $9 = 900 cents
Total number of stamps is 60 -> t + f = 60
Total value of stamps  is    -> 10*t + 15*f
Christie returned $1.75      -> $1.75 = 175 cents
Value of the stamps is       -> 10*t + 15*f = 900 - 175 = 725

We have now two equations with two variables (unknowns):

   t +    f = 60
10*t + 15*f = 725

If we solve first equation for t, we get -> t = 60 - f
Now replace t in the second with the expressed t from the first equation

10*(60-f)  + 15*f = 725
600 - 10*f + 15*f = 725
600 +  5*f        = 725
       5*f        = 125
         f        = 25

t = 60 - 25
t = 35

check: 10*35 + 15 * 25 = 350 + 375 = 725, OK