Question 1055575
Instead of DAY and MON, I'll use X and Y. 
{{{X+Y=180}}}
{{{X=4Y-20}}}
Substituting,
{{{(4Y-20)+Y=180}}}
Add 20 to both sides
{{{5Y-20+20=200}}}
{{{5Y=200}}}
Divide both sides by 5,
{{{Y=40}}}
Then,
{{{X+40=180}}}
{{{X=140}}}
So m<DAY=140 and m<MON=40.