Question 43: Two classes 7A and 7B go to work planting trees. Assume that the ratio of trees planted by class 7A to class 7B is \(0.8\) and that class 7B grows more trees than class 7A is \(20\). Calculate the number of trees class 7A can grow.

Let the number of crops grown by classes 7A, 7B in the order \(x\) and \( y\) \((x,y \in \mathbb {N^*})\).

According to the problem, we have \(\dfrac{x}{y}= 0.8=\dfrac{4}{5}\)

\( \Rightarrow \dfrac{x}{4} = \dfrac{y}{5}\) and \(y – x = 20\)

Thence inferred:

\(\dfrac{x}{4} = \dfrac{y}{5} = \dfrac{{ y- x }}{{ 5 – 4 }}= \dfrac{20}{1}=20\)

Therefore:

\( x = 20.4 = 80\) (satisfied)

\( y = 20.5 = 100\) (satisfied)

So the number of trees planted in class 7A is \(80\) trees.