The first 3 common multiples of 5 and 15 are 15, 30, and 45. Remember that the number itself is also a multiple of any number, since you can multiply it by 1.
5 - 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100
15 - 15, 30, 45, 60, 75, 90, 105
Hope this helps!
Let x be the degree of angle a.
Angle b: 20+3x
Angle c: 55+x
We know that all angles add up to 180 degrees.
180=x+(20+3x)+(55+x)
180= 5x+75
180-75=5x
105/5=x
21=x
Angle a: 21 degrees
Angle b: 20+3(21) => 83 degrees
Angle c: 21+55 => 76 degrees
Hope I helped :)

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Every triangle has 180 degrees when you add there angles so you know that they will add up to 180
they are right triangles so the right angle has 90 degrees
so for the yellow triangle
90+45+x=180
135+x=180
45=x
we can tell where the purple and yellow triangle touch is 90 so 45+x=90 to find that angle which is 45
and the other one is 45
so there are two 90 degree angles and three 45 degree angles.
I hope that made sence