I planted a 3 ft tree that has grown the same amount each year.at the end of the third year , the tree was 1/5 taller than it was at the end of the second year .the tree is now 18 feet tall. How many years ago did I plant the tree?

Question

1981 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2018-07-20T10:10:57

15 years ago

let #x# = the growth per year in feet

at the end of year 2, height
#h_2 = 3 + 2x# .....(1)

at the end of year 3, height
#h_3 = (3 + 3x) # ....(2)

but we are also told that #h_3 = h_2 +1/5*h_2#

#h_3 = (3 + 2x) + (0.2*(3 + 2x))#

#h_3 = 1.2*(3 + 2x)#

#h_3 = 2.4x + 3.6# ....(3)

combining eqns (2) and (3) (both equal to #h_3#)

#3 + 3x = 2.4x + 3.6#

# 3x - 2.4x = + 3.6 - 3#

#0.6x = 0.6#

# x = 0.6/0.6 = 1# ft/year

Total growth since planting #= 18 - 3 =15# ft

At 1 ft/year this has taken 15 years