Question 1072899
x, y, z, the number of 10 cent, 20 cent, 50 cent coins


{{{system(x+y+z=40,10x+20y+50z=840,z-y=1)}}}


{{{system(x+y+z=40,x+2y+5z=84,z-y=1)}}}



Sub for z
{{{system(x+y+y+1=40,x+2y+5y+5=84)}}}


{{{system(x+2y=39,x+7y=79)}}}

Subtract first from second equation
{{{5y=40}}}
{{{highlight_green(y=8)}}}----------the 20 cent coin count


You can find the other two coin quantities.