let one pipe take x min
then other take x+3 min
two pipes together can fil cistern in 40/13 min
two pipes together can fill 13/40 part of cistern in 1 min
one pipe can fil 1/x part of cister in 1 min
other pipe can fill 1/(x+3) part of cistern in 1 min
a/c to ques.
1/x +1/(x+3) =13/40
(2x+3)40=13(x2+3x)
80x+120=13x2 +39x
0=13x2-41x-120
0=13x2-65x+24x-120
0=13x(x-5) +24(x-5)
0= (x-5)(13x+24)
either, x=5 or, x= -24/13
taking ,x=5
so, first pipe will take 5 mins. whereas, other will take 8 mins.