b,$5 × (x + 35) - 15 = 25 × 15 - 35 : 7$
$5 × (x + 35) - 15 = 25 × 15 - 5$
$5 × (x + 35) - 15 = 370$
⇒ $5 × (x + 35) = 370 + 15 = 385$
⇔ $(x + 35) = 385 : 5 = 77$
⇔ $x = 77 - 35 = 42$
c, $720 : [41 - (2 × x - 5)] = 40$
$41 - (2 × x - 5)] = 720 : 40 = 18$
$2 × x - 5 = 41 - 18 = 23$
$2x = 23 + 5 = 18$
⇔ $x = 18 : 2 = 9$
d, $3x + 5x + 7x + ... + 19x = 297$
$(3 + 5 + 7 +... + 19)x = 297$
$[(3 + 19) × [(19 - 3) : 2 +1] : 2]x = 297$
$99x = 297$
⇔$x = 3$
e, $($$\frac{1}{1 × 2}$ + $\frac{1}{2 × 3}$ +...+$\frac{1}{19 × 20}$$)$ $: x =$ $\frac{9}{10}$
$1 - $$\frac{1}{20}$ $: x =$ $\frac{9}{10}$
$\frac{19}{20}$ $:$ $\frac{9}{10}$ = $x$
⇔$x =$$\frac{19}{18}$
f, $0,5 + (x -$ $\frac{5}{12}$) $:$ $\frac{1}{2}$ $=$ $\frac{9}{2}$
$(x -$ $\frac{5}{12}$) $:$ $\frac{1}{2}$ $=$ $\frac{9}{2}$ $- 0,5$
$(x -$ $\frac{5}{12}$) $:$ $\frac{1}{2}$ $=$ $\frac{8}{2}$
$(x -$ $\frac{5}{12}$) = $\frac{8}{2}$ $×$ $\frac{1}{2}$ = $2$
$x =$ $2 + $ $\frac{5}{12}$ = $\frac{29}{12}$
g,$1, 23 × x + 1,02 × x = 15, 75$
$(1,23 + 1,02)x = 15,75$
$2,25x = 15,75$
$x = 15,75 : 2,25 = 7$
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