Why 3u -18=15p?And why 10u-10p=15p?

(1) x 3 means for the entire equation 1, every term is multiplied by 3. 1u+6 = 5p becomes 3u + 18 = 15p. Same for (2) x 5. Every term in equation 2 is multiplied by 5. This method is called simultaneous equations, in this case, using substitution.

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