Answer:
Zak had 61 cherries at the start.
Step-by-step explanation:
Let the number of cherries be 'x'.
Given:
Number of cherries given to Tim = 18
Number of cherries given to Janet = 18
Number of cherries left = 25
Total number of cherries given = Number of cherries of Tim + Jane = 18 + 18 = 36
Number of cherries left = Total number of cherries - Total number given = 
Now, as per question:
Number of cherries left = 25
Therefore,

Adding 36 both sides, we get:

Therefore, the total number of cherries that Zak had at the start were 61.