Remark
This is a nice problem especially if you like blue jays! They actually do things like this.
Equations.
Let the number of trees = T
Let the number of acorns = A
20*T = A + 4 Read that carefully. There are 4 acorns left over. The number stored = A. The left over ones make 4 more than were stored.
Second equation
18*T = A - 4 Here we have 4 less than could be stored to full capacity.
Solution
20T = A + 4
18T = A - 4 Subtract
2T = 8 Divide by 2
T = 8/2; T = 4 Answer
20T = A + 4
20*4 = A
80 - 4 = A
A = 76 Answer
Answers
The number of Trees = T
The number of acorns = 76