Question

D varies directly with t. D is 105 when t is 3.. Find D when t is 8.. A] 35. B] 280. C]841

Answer 1

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 <===

Answer 2

D = kt 

solve for k when D= 105 and t = 3 

k = 35 

So, 

D = 35t 

when t is 8 

D = (35)(8) = 280