5x70=5x7tens.
60x4=6tensx4.
7x30=7x3tens.
90x3=9tensx3.
3a+7+2a and 7a+5. These expressions are not equivalent. 5a+7 does not equal 7a+5.
25%
If you are buying 2 shirts for the same price, you have to calculate the cost of the shirt at half price first (1 piece of the pie) and then add it the original price (2 equal pieces of the pie since you just divided that to the price of the second shirt). That would be considered 3 out of 4 pieces off the pie or 75%. Your savings would be 25%.
