Let 1 man's 1 day work = x and 1 boy's 1 day work = y. Then $6x+8y=1/10$ and $26x+48y=1/2$. Solving these equations, we get $x=1/100$ and $y=1/200$. Work done by 15 men and 20 boys in 1 day = $15/100 + 20/200 = 1/4$. So, they will complete the work in 4 days.