Find (a) sin (s+t) given cos s=4/5 and sin t=-12/13 and s and t in quadrant IV

The first week samantha learned to read braille, she could read 10 words per minute. in the second week she increased her speed by 20%. in the third week she increased her speed again by another 25%. how many words could she read per minute by the third week? hurry and tell me!