find the equations that tell their height
starting+growth=current height
starting=1
growth=2 times number of months grown
so
1+2m=h
another one is 11 and grows at 1 per month
11+1m=h
when will heights be equal?
set them equal and solve for the numberof months
1+2m=11+1m
minus 1m both sides
1+m=11
minus 1
m=10
answer is 10 months