Here is your answers:
Question one:
100% + 15% = 115%
Using the following formula:
(115%)(n)=20cm
Change the percent sign to 0.01 and solve:
115(0.01)(n)=20cm
1.15n=20cm
n= 20cm÷1.15
=17.3913043
Question two:
240cm to 312cm
312-240=72
72 difference
100%÷72
It would be a 30% increase in height:
=30%
Hope this helps!