The frog jumps 3 steps up, then 2 steps down.
After 1 double-jump, the frog has reached step 1.
After 2 double-jumps, the frog has reached step 2.
...
After 12 double-jumps, the frog has reached step 12.
But the next jump will be a three-step jump, which will take it to step 15. Finished!!
Total number of jumps is: 12 double jumps and one single three-step jump.
Number of jumps is: 12*2 + 1 = 24 + 1 = 25