36000 commission so we know for sure atleast 100 machines were sold ... eliminate A
30000 commission for 100 machines
so now we have to find out how many machines were sold so as to get 6000 (36000 - 30000) commission
for each machine sold after 100 machines the commission is 4% so we have to find 4% of what number is 6000
4/100*x = 6000 => x=150000

total value of sale above 100 is 150000
total number of sales is 150000/10000 = 15

so in all 100 + 15 = 115 machiens were sold

0,03*100*10000+0,04*10000*(x-100)=36000
solving for x we get x=115

the trick here is to make formula