To find the number of red bricks used, you can create a part to part to whole ratio from the simple ratio of red to grey brick they give.
Then, use the total to create an equivalent ratio with the actual total number of bricks (140).
5 red    100 red
2 grey =Â 40 grey
7 total   140 total
Each time, the simple ratio was multiplied by 20 to get the actual totals for each color.
There were 100 red bricks used.