Let
x---------> time in minutes used by Eric to study
y---------> time in minutes used by Kendra to study
z---------> time in minutes used by Geraldo to study
we know that
x=3y------> equation 1
x=0.5z
z=2x--------> equation 2
y=20 minutes --------> equation 3
substitute equation 3 in equation 1
x=3*20=60 minutes
find the value of z in equation 2
z=2*60=120 minutes
therefore
the answer is
The time in minutes used by Geraldo to study was 120 minutes