Question

using cross multiplication solve these

Answer 1

upstream u = b - c
downstream d = b +c
b = speed of boat
c = speed of current

24/u + 28/d =6
24 d + 28 u =6ud.........1
30/u + 21/d = 6.5
30d + 21u =6.5ud .........2
1 ÷ 2
(24 d + 28u) / (30d + 21u)=6/6.5
13(24d + 28u) =12(30d +21 u)

312d + 364u = 360d + 252u
364-252 u = 360 -312 d
112u = 48 d
u: d = 48 : 112
u:d =24: 56 = 6: 14 =3:7
so b -c /b+c = 3/7
7b -7c = 3b + 3c
4b = 10 c
b = 5
c = 2