Question 519603
You are right!
You need a second equation in which you equate the total value of the coins to the total sum given i.e. $7.60
Note that each quarter is worth $0.25 and each dime is worth $0.10, so...
{{{0.25(q)+0.10(d) = 7.60}}}
So now you have a system of two equations in two unknowns (q and d).
Rewrite the first equation d+q = 40 as d = 40-q and substitute into the second equation to get:
{{{0.25q+0.10(40-q) = 7.6}}} Simplify and solve for q.
{{{0.25q+4-0.1q = 7.6}}}
{{{0.15q+4 = 7.6}}} Subtract 4.
{{{0.15q = 3.6}}} Divide by 0.15
{{{q = 24}}} and {{{d = 40-24}}} {{{d = 16}}}