Question 98406
X*3X/(X+3X)=4 X= THE TIME FOR THE FASTER ROOFER.
3X^2/4X=4 NOW CROSS MULTIPLY
3X^2=16X
3X^2-16X=0
X(3X-16)=0
X=0 A NON ANSWER.
3X-16=0
3X=16
X=16/3 DAYS FOR THE FASTER ROOFER.
3*16/3=16 DAYS FOR THE SLOWER ROOFER.
PROOF
16/3*16/(16/3+16=4
(256/3)/[(16+16*3)/3]=4
(256/3)/[(16+48)/3]=4
(256/3)/(64/3)=4 
NOW INVERT THE DENOMINATOR & MULTIPLY
(256/3)(3/64)=4 CANCEL OUT THE 3s.
256/64=4
4=4