Tank B contained 3 times as many guppies as Tank C. When 10% of the guppies in Tank B and 40% of the guppies in Tank C were transferred to Tank D, Tank D had 84 guppies, which was 20% more guppies than before. How many more guppies were there in Tank D than Tank C in the end?

	Tank B	Tank C	Tank D
Before	3 u	1 u	5x0.7 = 3.5 u
Change	- 0.3 u	- 0.4 u	+ 1x0.7 = + 0.7 u
After	2.7 u	0.6 u	6x0.7 = 4.2 u

Number of guppies that were transferred from Tank B to Tank D
= 10% x 3 u
= ¹⁰₁₀₀ x 3 u
= 0.3 u

Number of guppies that were transferred from Tank C to Tank D
= 40% x 1 u
= ⁴⁰₁₀₀ x 1 u
= 0.4 u

20% = ²⁰₁₀₀ = ¹₅

Some guppies from Tank B and Tank C were transferred to Tank D.  The total number of guppies transferred from Tank B and Tank C into Tank D is the same.

Total number of guppies transferred from Tank B and Tank C into Tank D
= 0.3 u + 0.4 u
= 0.7 u

4.2 u = 84

1 u = 84 ÷ 4.2 = 20

Number of more guppies in Tank D than Tank C
= 4.2 u - 0.6 u
= 3.6 u
= 3.6 x 20
= 72

Answer(s): 72