The amount of DNA in the ${\mathrm{G}}_2$phase is 80pg. Calculate the amount of DNA in the ${\mathrm{G}}_1$phase.

During the S phase the number of chromosomes remain the same but the amount of DNA doubles. The ${\mathrm{G}}_2$phase comes after S phase. Therefore, ${\mathrm{G}}_2$ phase also has twice the amount of DNA.
The ${\mathrm{G}}_2$phase has 80pg DNA. Therefore, the ${\mathrm{G}}_1$phase will have 80/2=40pg DNA.