One cubic meter of air contains 0.21 m2 of oxygen. How many cubic meters of oxygen are there in a 140 m2 room?

You need to solve the problem with interest. To do this, we will use the proportioning. Let’s denote by x the percentage of oxygen. Let’s designate the whole room as 100%.
Let’s make the following proportion:
1 m = 100%;
0.21 = x%;
Let’s solve this proportion using the method of diagonals, multiplying each diagonal with each other.
1 * x = 0.21 * 100;
Let’s sort this equation by performing the possible multiplication actions:
1 * x = 21;
Find x:
x = 21%.
Second proportion:
140 = 100%;
x = 21%;
x = 29.4 m
Answer: 29.4 m ^ 3.