If D varies directly with t and D is 105 when t is 3, find D when t is 8

If it varies directly, then u can make a proportion.

105/3 = d/8

cross multiply

(3) (d) = (105) (8)

3d = 840

d = 840/3

d = 280 <===

D = kt

solve for k when D = 105 and t = 3

k = 35

So,

D = 35t

when t is 8

D = (35) (8) = 280

105/3=35

35*8=280

so it would be 280.