ok so i have this rate of change problem, driving me mad . its probably because of my algebra, but lets see

it is the altitude of a triangle decreases at a rate of 2cm/s while the base increases at a rate of 4 cm/s. how fast is the area changing when the altitude is 25 cm and the base is 10 cm.

what i have is dh/dt is 2cm/s and db/dt is 4

da/dt | b=10, h=25

a= 1/2 bh

a=bh/2

da/dt= ((db/dt*h + dh/dt*b)2 - (0)bh )/2

after canceling the 2's and subing in i get da/dt=(4)25+2(10)

da/dt = 120 cm^2/s

the answer says its 40 cm ^2/s not sure what im doin wrong here