i have doubt in word problem

Question

two vessels contain mixture of milk and water in the ratio 5:2 and 3:1 respectively. the ratio of milk and water in the new solution if two mixtures are mixed in equal amt 

 

answer

5+2 and 3+1 =7&4

lcm is 28 

 

why took lcm and how we convert it in 28 litre because 7and 4 are ratio  

Answer 1

5:2 and 3:1 are the ratios

5+2=7 and 3+1=4

lcm of 7 and 4 is 28

28 is the smallest number that can be expressed in a ratio of 5:2 and 3:1

28/7=4

4*5=20 and 4*2=8, 20 L milk and 8 L water

28/4=7

7*3=21 and 7*1=7, 21 L milk and 7 L water

add 28 L and 28 L to get 56 L

20 L milk +21 L milk=41 L milk

8 L water +7 L water=15 L water

the new ratio of milk to water is 41:15