Explain how using basic fact can find 10x20x30x40 mentally

Which number produces a rationale number when multiplied by 1/5

(x-5)1/2+5=2 what is a possible solution?

I'm at a bookstore with $\$35$ in my pocket and a $20\%$-off coupon. what is the cost of the most expensive book i can buy with my money and the coupon? (assume that there are books of every possible price in the bookstore. it's a large bookstore! )