25. 3y+15
26. 15k+20
27. 21n+7p
28. 60w+20x
Also, you got number 19, number 21, and number 24 wrong.
D+D+D+D+D = 5D, not D⁵
S+S+S = 3S, not S³
N+N+N+N = 4N, not N⁴
Remember, repeated addition, (n+n+n) is 3n. Repeated multiplication, (n•n•n) is n³.
Hope this helps.