Jack broke apart 5 times 216 as ( 5 times 200) + ( 5 times 16) to multiply mentally. what strategy did he use

7. if the value of x varies directly with the value of y, and x = 3 when y = 21. what is the valu y, and x = 3 when y = 21. what is the value of x when y = 105?

Can someone check if i did this right since i really want to make sure it’s correct. if you do you so much

The two square pyramids are similar. find the total volume of both pyramids if the ratio of their surface areas is 9/16