A fruit seller sells only guavas and lemons. The ratio of guavas to lemons is 3 : 4. A total of 120 fruits are added. The number of guavas becomes 3 times as many as before and the number of lemons becomes 2 times as many as before. How many fruits are there at first?

	Comparing guavas at first and in the end	Guavas	Lemons	Comparing lemons at first and in the end
Before	1x3 = 3 u	3 u	4 u	1x4 = 4 u
Change		+ 120
After	3x3 = 9 u	9 u	8 u	2x4 = 8 u

The number of lemons at first is repeated.  Make the number of lemons at first the same.  LCM of 1 and 4 is 4.

The number of guavas at first is repeated. Make the number of guavas at first the same. LCM of 1 and 3 is 3.

Number of fruits at first
= 3 u + 4 u
= 7 u

Number of fruits in the end
= 9 u + 8 u
= 17 u

Number of fruits added
= 17 u - 7 u 
= 10 u

10 u = 120

1 u = 120 ÷ 10 = 12

Number of fruits at first
= 7 u
= 7 x 12
= 84

Answer(s): 84