Answer
a)
$x+y =15$
$.01x+.25y = 4.35$
$x=-2.5$, $y=17.5$
b) No, it's not reasonable. You can't have a negative number of coins (or half of a coin).
Work Step by Step
$x+y =15$
$.01x+.25y = 4.35$
$x+y =15$
$x+y-x =15-x$
$y = 15-x$
$.01x+.25y=4.35$
$.01x+.25*(15-x)=4.35$
$.01x+3.75-.25x=4.35$
$-.24x+3.75=4.35$
$-.24x+3.75-3.75=4.35-3.75$
$-.24x = .6$
$-.24x/-.24 = .6/-.24$
$x = -2.5$
$x+y = 15$
$-2.5 +y = 15$
$-2.5 + y + 2.5 = 15 +2.5$
$y= 17.5$
