You're told that company A has 400 machines of which 20% are high-speed: that is 80. Company B has X machines in total (this is the quantity you're being asked to determine) of which 60% are high-speed : that is, 60X/100. The total is 400+X machines of which 44% are high-speed: that is, 44(40+X)/100. But the total number of high-speed machines is also 80 (from company A) plus 60X/100 (from company B). So we have an equation 44(X+400)/100 = 80 + 60X/100. You now have to solve for X .