7· 15 can be written as
5(7 + 10)
10(7 + 5)
7(10 + 5)

For the answer to the question above, 
An easy trick can be to just find the product of 15*7. Then with that product see which product equals to the ones written on the answer choices.

the answer is 7(10+5)
I hope my answer helped you.

38+105= 143, 39+104=143, and you will find then adding the numbers on opposite sides, moving inwards yields the answer 138 every time. because of this, you’re going to multiply 138 times the number of pairs, which is 34. therefore, 34*138= 4692(i’m sorry if it’s the wrong numbers, but it’s the right concept)