These are the types where you are asked to create a special mixture of two substances of different value.

Total weight = 20.  Total value = $20\times1=20$

$x+y=20$

$.75x+1.25y=20$

$simlt(1,1,20,.75,1.25,20)$

$x=10 y=10$

$simlt(1,1,100,5,10,700)$

60