A certain kudzu plant was 11 feet long on the day it was planted, and the length grows
by a constant amount every day (including the first day it was planted). If this plant
grew 40% between the fourth day after it was planted and the tenth day after it was
planted, how many feet per day does it grow?

answer 1 foot per day.

My logic : it increases by a constant amount(say d) each day. so if 1st day it is a, on subsequent days it will be a+d, a+2d, a+3d ....

therefore, on 4th day it will be a+3d and on 10th -> a+9d

and,
a+9d= 1.4(a+3d)
a+9d = 1.4 a + 4.2d
0.4a = 4.8 d
a=11
therefore 4.8d= 4.4
so d = 11/12 foot

where am i going wrong? can someone help please....
thanks,
pooja