We can solve this by writing down all of the variables we know. We will call the distance traveled on Saturday, x. The distance traveled on Sunday will be y.
x = distance traveled saturday
y = distance traveled sunday
We are told that on sunday he rode 3 miles more than 2/3 the distance on saturday. We can write a new formula.
y = (2/3)x + 3
We also not the total distance travelled, x + y = 43, now we solve for x.
x + y = 43
x + (2/3)x + 3 = 43
5/3x = 40
x = 24 miles
y = (2/3)(24)+3
y = 19 miles
Therefore, Mario biked 24 miles on Saturday and 19 miles on Sunday which gives us the total of 43 miles for the whole weekend.