Answer:
fail 0-x. mid values. =X/2
pass x-a. =(x+a)/2. pass no.=250-35-15-23=177
credit a-y. =(a+y)/2
distinction y-100. =(y+100)/2
mean=1/250*(x/2)×23+(x+a)/2*177+(y+a)/2×35+(y+100)/2×15=64
1/500[23x+177x+177a+35a+35y+15y+100*15)=64
200x+212a+50y+1500=32000
variance=[(X/2-64)^2*23+(x+a,/2-64)^2*277+(a+y/2-64)^2*35+(100+y/2-64)^2*15]/250=(8.6)^2
I've made two equations that has to be simplified.
probably a nother data has to be given since we have 3 variables to solve for(X,a,y)
