yes, there is a fast way to get the first one.
Calculate Shana's average, which is 450/5=90
In order to keep her average the same, she needs a 90
as below 90 will drop her average and above 90 will raise it.
Apparently that will necessarily raise Mina's average by 1,
so that information was in fact irrelevant as only 90 will keep Shana's average steady.
For 2) you need