Question 1100750
Let k be the speed of lorry B, and k+5 be the speed of lorry A. Then:
3120/k  -4=3120/k+5
3120(k+5) -4(k)(k+5)=3120k
3120k+15600-4kČ-20k=3120k
4kČ+20k-15600=0
kČ+5k-3900=0
(k-60)(k+65)=0
k=-65 or 60
We get a positive value of 60 kph for the speed of lorry B; and 65 kph for lorry A
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