The key to word (story) problems is to correctly translate the words into concise, precise math expressions (variables and operations).
First, assign variables. Try to include the one we are asked to report.
Let T = total length of entire practice in hours
Let A = time stretching in minutes
Let B = time scrimmaging in minutes
Let D = drills in minutes
Then, translate:
"10 minutes of stretching" means A = 10 (eq1)
"2/3 of the entire practice scrimmaging" means (2/3)*T = B (eq2)
(note: "of" means multiply)
"remaining practice time working on drills of specific skills"
means D = T - A - B (eq3) (total minus others)
"100 minutes of stretching and scrimmaging" means 100 = A + B (eq4)
"how long in hours is the entire practice" means report T
Now, the easy math:
Put the value of A from eq1 into eq4 to find P
100 = 10 + B
90 = B (subtract P from both sides)
Now, we can put that value of B into eq2 to find T
(2/3)*T = 90
T = (3/2)*90 (multiply both sides by (3/2)
T = 135 minutes = 2 hours 15 minutes
But, we have not found D (but then, the problem didn't ask us to). Let's find it to check our answer (very important).
Put T,A, and B into eq3. D = 35.
Final check:
Is 10 + (2/3)135 + 35 = 135 ? yes.
Is (2/3)135 = 90 ? yes