Question 111232
Your first and third equations are correct, but the second one should be {{{l=3c}}}.
:
That makes your first substitution equation: {{{600=(l/3)+l+l-30}}}
{{{630=7l/3}}}
{{{l=270}}}
:
So, 270 long blocks, 90 cubes, and 240 flats.
270 is three times 90 and 30 more than 240, and 270+90+240=600, so the answer checks.