We all take more time to solve a system of two or more equations than an equation with a single unknown. So the following will avoid system of 2 equations for this problem... by pre-substitution.
R=number of red apples
80-R=number of green apples
Equation is:
R/5 =3(80-R)
Cross multiply and solve for R
R=15(80-R)
R+15R=1200
R=1200/16=75
So Mr. Teo has 75 Red apples (and 5 green apples)