As per Question,
Break-even unts = $800000 / ($4 x (1 - .60)) = 800000 / (4 x .40) = 500,000
{note: 60/100= 0.06}
that means....,
Stuart Company sells 500,000 units @ $4 per = $2,000,000 in sales.
...i.e.., Break-even dolalrs = 4 x 500000 = $2,000,000
Margin of safety = (2500000 - 2000000) x .40 = $200,000
....i.e.,Margin of safety ratio = 200000 / 800000 = 25%
Graph: Approximate - not real !